An old man riding his bike along the street of Tokyo. Photo credit to my Amore.
Official Bike Laws
Basic rules of the Road
Cyclists ride on the left hand side of the road.
Riding dangerously, failing to stop at a stop light, or riding with broken brakes carries a maximum penalty of a ¥500,000 fine and/or three months in prison.
Biking under the influence of alcohol is forbidden and carries a maximum penalty of five years in prison and a ¥1,000,000 fine.
Cycling on sidewalks is forbidden, except where indicated by shared sidewalk signs.
An overview of traffic law as well as a guide to Japanese traffic signs can be found here (pdf). Alternately, there is a very thorough guide to the rules of the road that can be purchased through the Japanese Automobile Foundation.
Other Bicycle Laws
All bikes are required to have a bell and a headlamp (if riding at night)
It is illegal to ride tandem bicycle (except in Nagano) [Read more about the tandem riding rule]
Also illegal (and in line with the previous): riding with a passenger (with the exception of a child below the age of six). Double riders could face a ¥500,000 fine
A cyclist may carry one child under the age of six in a designated child seat
Children under the age of 13 years must wear a bike helmet
Bicycles must be registered in the owner’s name at the prefectural police department
As you noticed, it is not illegal in Japan to get very comfy while riding your bike :D
Happy Cycling and Stay Safe!