First, ensure that you are familiar with the rules of the road in Japan – the most obvious being that traffic drives on the left side of the road. The Japan Automobile Federation (JAF) issues a book called “Rules of the Road” in five languages, which is designed to prevent traffic accidents involving foreign residents in Japan and promote safety on the road. The price is 1,000 yen per copy, but first, you might want to ask around amongst JETs to see if they have a copy lying around that you could have or borrow.
To drive legally in Japan you will need to have either an International Driver’s Permit (IDP) plus valid home country license, or a valid Japanese driver’s license. Your IDP is valid for one year from either your date of entry into Japan or the date you got it (written on the IDP), whichever comes first. This means you cannot leave the country and just renew your IDP when it expires.
Even if you don’t plan on owning a car or driving in Japan, consider getting an IDP to use in case you want to rent a car.
Please be sure to confirm the expiration date, and if you plan to stay in Japan and drive after this time plan to get a Japanese license. Do NOT drive in Japan without a valid license; it is a serious offense and could result in a very expensive fine and/or revocation of your visa, which equals deportation.
If you will need to get a Japanese license, start preparing as early as possible. For those with a valid foreign license, the conversion process can sometimes take one month or more, even for those from Commonwealth countries. Visit the license conversion section for more details. If you want to learn about the process of getting a Japanese license from scratch, read the driving school section.
The National Police Agency have a English language booklet describing the most basic rules of the road here. And you can get a bit more information about traffic safety, especially in regard to pedestrians and cyclists, here.
Drinking and Driving (+ Cycling)
Finally, remember that Japan has a zero tolerance law with regards to alcohol and driving (including bicycles!). The legal BAC level is 0.0%. The police have been known to set up random checkpoints, and when drunk drivers are apprehended, it is a major story on the evening news. This is especially true when those apprehended are public servants, and even more so when they are foreign.
Fortunately, Japan has a system of two-driver taxis called “daiko” (代行) where a second driver can drive your vehicle home for you. The fare is negotiated in advance, and is usually not too far off what a regular taxi would cost, and in many cases is cheaper.