"Safe Riding Rule Book for Bicycle Users"

Riding double is illegal.

Limitation on riding capacity

Paragraph 2, Article 57 of the Road Traffic Law and Article 11 of Osaka Prefecture's Road Traffic Rules

Punishment: A fine of 20,000 yen or less
Riding capacity of a bicycle is limited to one person, that of a tricycle is depending on how the tricycle is equipped except in the following conditions.
 When a cyclist is over 16 years old,
 ・ riding a bicycle with a child under 6 years old in a child seat;
 ・ riding a two-child seated bicycle with two children under 6 years old in each seat;  
 ・ riding a bicycle carrying a child under 4 years old piggyback tied with a string or things like that (except when riding a two-child seated bicycle with two children in each seat.)
 
※ When riding a two child-seated bicycle, be sure to use it properly in a safe way such as  releasing a safety lock.
You are recommended to use a two-child seated bicycle with a SG Mark or a BAA Mark. Those marks mean that bicycles are manufactured in compliance with the Japan's safety standard so you can use these bicycle safely.

Regular bicycle
A two-child seated bicycle
We must ride a bicycle properly for our children's safety.
BAA Mark SG Mark