Question re 90-day entry permit

If an American enters Japan on a 90-day entry permit, do I need to show an outbound flight ticket or do you just need your passport?

I believe it is the check-in personnel back in the US, which will make sure that you have either a return ticket or a visa for living in Japan before you board the plane. At the Japanese immigration itself, I don't think they check for return tickets.

Uji is right. As Japan does technically require proof of return/onward travel, airline check-in staff may question you if you are travelling on a one-way flight, as they are liable if you are turned away from Japan.

Japan Immigration/customs are unlikely to check unless they decide to question you thoroughly for some reason- for example, they suspect you of coming to Japan to look for work.

I've never been asked to show an outbound ticket after travelling in and out of Japan some six times in 2 years. I've even travelled to Japan on a one-way ticket of of Bankok though I was required by the airline to sign a form indemnifying them in the event that I was turned back. Technically I believe you could be asked to show evidence of your outward travel - this is the case in other countries as well.