Upvote:8
In order to board your flight to Japan, you will need to prove to your airline that you have sufficient documentation to be able to complete your journey. Airlines use databases to check what documentation is required. If you complete the form here with your information you will see that a visa is not required for transit if you stay in the transit area. However, as you say, you will need to leave the transit area in order to check in for your second leg. The form above gives this guidance:
Transit visa is required if the passenger leaves the transit area of the airport.
Valid visas in expired passports will be accepted provided officially bound to a second valid passport.
Passengers transiting through Japan may be granted an entry permit at the discretion of immigration staff. They can not leave the area outlined by the staff member, and other restrictions may apply. Maximum stay of 72 hours.
So in fact, you will need a transit visa. The third paragraph offers an alternative, but Alitalia may not allow you to board anyway, since they may be fined if they deliver you to Japan with insufficient documentation.
In conclusion, if you need to leave the transit area, you ought to have a transit visa.