In Japan you can find bicycles everywhere, often you see them thrown away even though they are still perfectly all right to use. Many foreigners find this to an opportunity to avail themselves of a bicycle for free. Since it is just laying there beside the road and nobody cares..... take it home and do some repairs and "voila" you have you own bicycle.....what could be wrong with that?
Well, actually a lot.

You can be ARRESTED for riding on a bicycle that is not properly registered in your name

All new bicycles must undergo crime-prevention registration after they are purchased. Even if you are given a used bicycle from a friend or acquaintance you must re-register the bicycle. If you just leave the bicycle registered under the name of the previous owner you could be suspected of stealing it.

So you can not just pick up any bicycle and use it as your own. Be sure to buy one from either a store or a person and don’t forget to register the bicycle under your name.
This is called,

Bicycle (Crime-prevention) Registration Registration Fee: 500 yen (Most bicycle shops handle crime-prevention registration.)	SAMPLE SAMPLE

And speaking of laws,

did you know you can be ARRESTED for violating other bicycle laws as well?

According to the Daily Yomiuri (September 18, 2004), a rider who carries another person on a bike can be fined up to 20,000 yen．You can however ride with your child if you use a special child seat that can be attached to the bicycle and if the child is under 6 years old.

Cyclist who is caught talking on a cellular phone or using an umbrella while riding a bicycle could receive a maximum prison sentence of three months or be fined up to 50,000 yen.

From April, 2006 you can also be fined for riding on your bicycle while under the influence of alcohol and for not stopping at a red light and for not having your light turned on at night. The fine is up to 50,000 yen.
Bicycles are considered to be a subject of traffic law as much as an automobiles are. A cyclist who caused an accident that resulted a serious injury or a death may be charged in the same way as for an automobile driver.

It is illegal to park a bike at public places such as train station, bus terminal unless it is a designated area for it. Illegally parked bicycles may be removed, and will be disposed of after two months if not reclaimed.

You will very quickly notice that the people around you do not follow these rules, however, these are the rules that the police will enforce if it will become a police matter. So, be aware.

Bicycle riders must obey traffic laws since bikes are considered vehicles. That means riding on the left side of the road. Bikes can be ridden on sidewalks when indicated ("Bicycles and Pedestrians" sign) but pedestrians technically have the right of way.

When crossing the road (where most bicycle accidents happen)

By law, one can not ride a bicycle on pedestrian's crossing even if that crossing connects two pedestrians walks both of which permits bicycle ride. You must get off the bicycle and walk across while pushing the bicycle.

Pedestrian Crossing Zone	Pedestrian and Bicycle Crossing Zone	Bicycle Crossing Zone
You must get off your bicycle and push it across.	You can ride your bicycle across within a designated strip on the side of pedestrian crossing.	Bicycle crossing only.

Except when there is something blocking the way, bicycles should always be ridden on the left side of the road.
Bicycles may travel inside the pedestrian strip, except when it would seriously obstruct pedestrian traffic. Pedestrian strips marked with double white lines are closed to bicycles.