Hmmm. I don't have one but after talking to a friend apparently it's easy but very looooong. She said, if you can prove that you have driven in your country for more than 3 months since you got your license, it's an easy BUT looooong process. There's a document check, health check, knowledge test and depending on what country you are from, you might have to take a driving test on the course. To make the process smoother, she recommends bringing a friend who can interpret for you. Check out this website for more details: http://www.japandriverslicense.com/index.asp
Good luck!