[Sangetsu]

If you stay long enough that you have to renew your visa again, you would be wise to do it yourself. Get the paperwork you need one month before your visa expires. You'll need a form filled out and hanko'd by school or employer. If you are employed/self employed, you'll need an original copy of your tax form as well.

Take these papers and your passport to the immigration office. They'll give you an application to fill out, and a post card (you'll need to buy a stamp for the postcard, and payment stamps for the renewal application, these are available for sale in the same building). Once these items are filled out, they'll be checked, and then your passport will receive an "in process" stamp.

In a week or so the postcard will arrive in your mailbox telling you your visa is ready. Go back to the immigration office, get your new visa, and you are done. If you arrive early in the morning, the process should take no longer than 30 minutes for each visit. It's a much better option than letting someone else keep your passport for three or more weeks.