Speaking falsehood is sinful. Does it mean speaking is the truth is no sinful?

Speaking false is obviously sinful. Telling truth is not a sin. The next action which happen can be not good when speaking truth. So the best thing is to be quiet at those situations. If you are speaking false in order to get rid of obstacles it is far more valueble to tell truth and welcome the difficulty. Because karma for speaking false is very much dangerous than small problem you face in this life

It's not akusala (i.e. bad, unwholesome, unskillful) on its own to speak the truth.

But of course, as you have seen, U Eain Myanmar, it can be unpleasant: and therefore it's allowed and OK to not speak at all, first having checked (i.e. depending on) the other person or the assembly to which one speaks (if drawing from the Vinaya, which is a good guideline for everyone).

Dukkathā Sutta: Unpleasant talk

Bhikkhus, five kinds of talks are unpleasant varying from person to person. What five?...

As for the case of criticism, which is (if not selfishly motivated) a compassionate action, to be most effective one should not only speak out of compassion; it's also best to have no faults as well, to be free from faults; and to speak with digestible words.

If only speaking as a claim, like if one has a burden with another's disability, that should be not seen as a good cause to speak out even the truth.

So does, "Don't speak out your mind just for your gains, and endure unpleasant things having fallen to you! For whom if not for yourself did you speak?", make you angry? Even a matter to do no more speaking with my person? If so, why?

The Buddha and the wise prefer truth over "harmony" or equanimity: and telling faults and making amends is very important:

Then the Blessed One addressed the monks, “These worthless men, having spent the Rains uncomfortably, claim to have spent the Rains comfortably.

“Having spent the Rains in cattle-like affiliation, these worthless men claim to have spent the Rains comfortably.

“Having spent the Rains in sheep-like affiliation, these worthless men claim to have spent the Rains comfortably.

“Having spent the Rains in heedless-affiliation, these worthless men claim to have spent the Rains comfortably.

“How can these worthless men undertake a vow of dumb silence, the undertaking of sectarians?

Therfore, only a fool, not a wise is not pleased when getting reasonable and compassionate treatment by a critic:

Regard him as one who points out treasure, the wise one who seeing your faults rebukes you. Stay with this sort of sage. For the one who stays with a sage of this sort, things get better, not worse.

Let him admonish, instruct, deflect you away from poor manners. To the good, he's endearing; to the bad, he's not.

Why, living in Burma, does one seeks for advices on global trade and exchange places!?

Perhaps you can approach good monks, possible not far, and ask.

There is a nice story of the Hermit and the Rabbit -- in regard of judging, intent and defilements of greed, hatred, delusion and fear that might effect one's intention.

“The Great Hermit Saves the Tiger’s Life” (PRBK 2001: 3, 1).54 Here, rabbit judges without bias the case of the great hermit and the tiger. This folktale describes a great hermit who saves the life of a tiger bitten while sleeping in front of a snake’s hole, but the ungrateful tiger wants to eat him anyway. Therefore, they go off to find a judge who will settle their dispute. The first judge is a jackal. His bias is partially caused by love or desire (chandāgati). He says to himself, “If I judge the tiger and lose the case, I won’t be able to depend on his power in this forest.” Not accepting this judgment they go to the second judge, a cow. He is prejudiced by fear (bhayāgati). He reckons that, “If I do not help the tiger, he will hate me and eat me.” Being dissatisfied with that judgment, they go to the third judge, a monkey. His bias is caused by hatred or enmity (dosāgati). He thinks that, “In the past, a man had fallen into a well and my father helped him, however that crafty man ate my father.” Disagreeing with that judgment, they go to the fourth judge, a buzzard, who is prejudiced by greed or desire (lophāgati). He thinks that, “Today I frequently get my food from the remains of the tiger’s meal. If I judge that the tiger should lose the case, he will be angry with me; and how can I get the food from him?” Being dissatisfied with that discernment, they go to the fifth judge, a tree spirit. He is prejudiced by delusion or stupidity (mohāgati). He decides that, “People walking to the forest and taking shelter always break and cut off the leaves so I will judge that the tiger wins the case.” The last judge is the rabbit who is independent and neutral (sugatigamanam). He orders the tiger to go back to where the original event had taken place and if the tiger remains unbitten by the snake, he wins. Rabbit was not intimidated by the tiger’s physical power. He judged wisely, excluding love, hatred, greed, and delusion. It is clear that these four concepts are taken from the Buddhist ideas of how to be a good ruler or king. That is to say the good king and ruler must conduct his duties according to them (Payutto 1996: 24). A comparison between Judge Rabbit and the other animals and supernatural judges—cow, wolf, buzzard, tree spirit, and monkey—indicates that only the rabbit is an impartial judge. He represents the wisest one.

At least my person likes to add that, unlike what's commonly believed, giving favour and pleasure is not a factor of awakening ... while sacca (truthfulness) is. As long a person prefers pleasure and binding over truthfulness, he/she is incapable of developing the eightfold path, even to develop metta: his/her heart would be still corrupted, and gravely attached and bound. Of course such might gain large gatherings, and a following, for a while, in the world, but not toward liberation.

That's again a reason why wise prefer a smart "opponent" over a foolish "friend".

So if one again draws a guide, in regard of relations, from Buddha's ways for his community:

He urged first to criticise wrong actions, made it to a duty; and second, since people got beaten up, allowed them to refrain from criticism, after testing the gathering, if guessing that they will not be able to speak truthfully.

In fact he said you can also consider the "power" by different actions to be taken, which may vary depending on the different numbers of people on the right side.

“Monks, I allow four or five to protest, two or three to voice an opinion, and one to determine, ‘This is not agreeable to me.’”

Note that when not speaking, the thought that "This is not agreeable to me." is most important: since tolerating unskillful deeds even by thoughts, is unskillful kamma.

On the other side it's also usual to ask for leave (unusual to request permission) before accusing someone of a fault. More on that, if liking to adopt, one finds in IV. Pavāraṇākkhandhako: The Invitation Khandhaka.

[Note: This is a gift of Dhamma, not meant for commercial use or other lower wordily gains by ways of exchange or trade]

[1] In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial (or: not connected with the goal), unendearing & disagreeable to others, he does not say them.

[2] In the case of words that the Tathagata knows to be factual, true, unbeneficial, unendearing & disagreeable to others, he does not say them.

[3] In the case of words that the Tathagata knows to be factual, true, beneficial, but unendearing & disagreeable to others, he has a sense of the proper time for saying them.

[4] In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial, but endearing & agreeable to others, he does not say them.

[5] In the case of words that the Tathagata knows to be factual, true, unbeneficial, but endearing & agreeable to others, he does not say them.

[6] In the case of words that the Tathagata knows to be factual, true, beneficial, and endearing & agreeable to others, he has a sense of the proper time for saying them. Why is that? Because the Tathagata has sympathy for living beings."

Abhaya Sutta

This page has extracts from a dozen suttas, which explain what "Right Speech" is.

Being "truthful" is an aspect, but not the only aspect, of Right Speech: for example speech should also be affectionate and spoken with a mind of good-will, beneficial, and spoken at the right time. Also polite, not divisive, and so on.