extending tourist visum

I will land in Tokyo the 21st of March and my retour ticket is set to the 15th of April. At immigration I'll get a stamp saying I should leave the country the 15th of April, prior to my ticket. But what if I feel like staying longer while in Japan ? How should I extend my tourist visum (stamp on passport) ? I know the maximum is 3 months and I will probably don't even stay that long, but if I get a passport check on the streets in Japan and the stamp says I should have already left the country, this would be bothersome right ?

I'll check my passport, but when I went as a tourist, I don't remember them stamping it with my return date; what's important is the day you arrive, as you have three months from that day to stay in the country.

I'm so sorry, for some reason I was convinced that I had such a stamp on my passport but I just checked it again and it's actually a stamp that says the date of landing and the date of leaving, 3 months after.

Anyway, case closed.