To answer the question posted in the title: yes you can. If you are admitted into the USA, you will get a “admit until date”. This date might be stamped into your passport and also will be accessible in your electronic I-94 record. Up until that date you can stay in the USA legally.
This is a convoluted world. If you lie to the CBP they will catch you, they are trained and experienced in catching liars and you won’t be admitted. But if you don’t lie and gets admitted andd something just comes up then sure you can stay longer…
I have actually been in very similar situation and have traveled many times to the US to visit my girlfriend through the Visa Waiver Program/ESTA (I am from Germany, but I don’t think that makes a big difference). The longest I stayed was about 2 months right after I finished university. I always told them how long I planned to stay and the purpose of my stay and it was never a problem. There is absolutely nothing for you to worry about.
When the CBP officer asks you about your stay. Give them a concise and truthful answer. No need to elaborate unnecessarily. If they want more information, they will ask you. When I first came, I felt like I had explain why I have a girlfriend in the US and why we had a long distance relationship etc. They won’t really have time to listen to your life-story and will probably just cut you off.
In my experience these are the type of questions they will ask you and the level of detail they expect:
A couple of tips:
Japanese citizens can travel to the USA on the Visa Waiver Program/ESTA, and I assume this is what you are intending to do.
When entering the USA on ESTA, no matter what you tell the CBP officer about the length of your intended stay, if they let you in you will usually be allowed to stay 90 days. Exceptions do happen, but they are not very common. This is more than twice as much as what you are planning. Therefore you have nothing to worry about — just tell the truth about 40 days, and stay up to whatever limit the CBP officer set (whether it is more, or even if it is less, though that is rather unlikely) if things go better than planned 🙂
Your problem statement sounds like the beginnings of a huge mess that would potentially take years to untangle. When talking to immigration, your best strategy is to always answer truthfully when asked a specific question. If asked for how long you’re planning to stay, answer the exact date. If asked whom you’re planning to visit, answer that you’re visiting your boyfriend. If asked what you’re doing in Japan – explain it concisely (work, school, business), as well as why taking off 40 days to visit the US won’t interrupt said activity. All you have to do is provide a short answer to the exact question asked – you don’t have to tell the immigration officers long stories about your fears, a simple “I’m visiting for 40 days” statement is perfectly sufficient.
What you’re describing is a routine scenario and 40 days isn’t particularly long for a tourist visit. Relax, say the truth, and enjoy your trip.
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