One note of caution--- I've stayed for a period of 180 days, on two seperate tourist visas, while there I've tried unsucessfully to understand the complicated visa situation--- it may be less complicated, but from phoning numerous embasies, consulate, etc, the answers I've recieved were curt and inefectual--- I've gotten no help from the ogainizations who could most help me---
Recently, I took a trip to the United States for a week, and went back to live in Japan for another 3 months--- unfortunately, I was denied entry, because they didn't feel I had a valid reason to be there---
Believe me, you DEFINITELY need a plan, I learned this AFTER I got there, the hard way, and from the fine people posting on this board, who were MORESO informative and helpfull than the institutions I phoned regarding visa information--- recently I spoke to the sister of my Japanese girlfriend who told me not to even bother with speaking to Immigration, as frankly, they wont help, and don't care.
Good luck--- you can do the visa run ONCE, but unless you have a good history of what you've been doing IN Japan, read, documentation of your attempt to get a different visa, etc etc, Immigration my disavow you entry (and probably will do so), thereafter.