The Hague Treaty, which requires nations to return children taken by one parent to the countries where they usually reside, went into effect in Japan on Tuesday.
The Diet last year approved the international treaty on child abductions after decades of pressure from the United States and other Western nations.
Japan was the only member of the Group of Eight major industrialised nations that had not ratified the 1980 Hague Convention.
Hundreds of parents, mostly men from North America, Europe as well as thousands of Japanese, have been left without any recourse after their estranged partners took their children away.
Unlike Western nations, Japan does not recognize joint custody and courts almost always order that children of divorcees live with their mothers.
Under the convention, which applies to children under 16, a central authority will be set up in the foreign ministry to take charge of locating children who have been removed by one parent following the collapse of an international marriage, and to encourage parents to settle disputes voluntarily.
If consultations fail, family courts in Tokyo and Osaka will issue rulings.
The law will, however, allow a parent to refuse to return a child if abuse or domestic violence is feared, a provision campaigners say is vital, but which some say risks being exploited.
The law will allow for parents who separated before its enactment to apply to get a child returned, but contains a provision stating that the application can be refused if a child has been resident in the country for a year or more and is happily settled.
Detractors say the lumbering pace of Japan's justice system, where cases can take months or even years to be heard, will reduce the chance of a foreign parent making a successful applicant to have their child returned.© Japan Today/AFP