If you are going to commute by bicycle in Japan, there are three basic approaches to the situation that you can take, depending on the type of commuting that you are looking to do.
Mamachari is Japanese for the “mother chariot,” and these bicycles often can been seen with child baskets on the front and back. More commonly, however, they are the bicycles of young students, old men, and everyone in between.
They are usually cheap, single-speed bicycles that come outfitted with the full compliment of rain and dirt avoiding gear, as well as baskets and racks for hauling things like bags and groceries.
Currently, in addition to my road bike, I own the cheapest mamachari money could buy from a local shop. The bike gets me around the city, but I would not take it further than 5km each way as it can be quite tiresome to ride.
The mamachari is almost never ridden in the street, but usually on Japan’s wide and bicycle-friendly sidewalks. Almost exclusively without a helmet. If it is raining you can generally stay dry by riding with an umbrella in your right hand.
Medium-distance: Mountain bike, mini-velo, or road-racer
If you are looking to go further than just around town, or just don’t like the setup of the mamachari, your next option would likely be some sort of mountain bike. Aside from mamachari, they are the most commonly seen bicycle on the roads here.
Once you get out of the city you will often see people riding this cycles in the streets or on the sidewalk (where there still is one). They often don’t include the same basket space as the mamachari so you will have to carry a backpack if you want to haul things.
Also options are the mini-velo and road racer, which are even more efficient than the mamachari or mountain bike. However, these are less common because of their price.
Long-distance: Road racer
If you are planning to do anything more than 20 mile round trips, I would suggest you pick up a decently geared road bike. Many hills you will encounter in Japan are severe enough that you will want a solid bike to help you get up them so that you don’t end up walking all the time.
If purchasing a bicycle in Japan, you can get decent road bikes for commuting or touring at many larger bike shops. While I often use specialty shops because I have been commuting on my racing bicycle, this isn’t really necessary for the average commuter.
Road bikes can be purchased as cheaply as 800-900 bucks for the lower end of the Bianchi line, which seems to be popular all across Japan. You will also find other American and European makers for sale up from that price.
If commuting with a road bike, I suggest getting yourself a comfortable and roomy messenger bag to carry your things since it is likely your bike won’t be doing the work for you.