More than 400 since 1994 when the U.S. Department of State's Office of Children's Issue's (OCI) was established. Japan is internationally known as a black hole for child abduction.
Where does this 400+ figure come from?
It comes from data provided by the U.S. Department of State. Representative Chris Smith reported at the May 24, 2011 hearing in the House Foreign Affairs subcommittee, “Japan has by far the worst record of all. It has not issued and enforced the return order for a single one of the more than 321 American children abducted there since 1994, when the recordkeeping began.”
321 was also the figure reported to us in our July 2011 meeting with OCI by then director Beth Payne along with then Assistant Secretary of State for East Asia and Pacific Kurt Campbell.
At this meeting we requested updated figures dating back to 1994. On August 25, 2011 parents of children kidnapped to Japan received a response stating, "As of August 19, 2011, that statistic is 268 cases involving 374 children."
How many U.S. children have been abducted within Japan?
It is difficult to obtain an exact number. Based on divorce and childbirth rates in Japan, estimates range from hundreds to 10,000.
Is the Government of Japan helping?
Japan consistently fails to return kidnapped children or provide access between the abducted child and parent. Court orders in Japan are unenforceable without the voluntary compliance of the kidnapping parent who has no incentive to comply. In a meeting with Embassy of Japan officials in May 2018 to discuss if there is any genuine path for Japan to reunite parents with kidnapped children, we were told, "Your access (to your children) depends on the mother and child's wishes." The Government of Japan has ceded all control to the kidnapping parent. They have also been caught providing advice on how to abduct and circumvent a Hague Abduction Convention return order.
 May 24, 2011 House FA Subcommittee hearing transcript, "INTERNATIONAL CHILD ABDUCTION: BROKEN LAWS AND BEREAVED LIVES", page 2, https://chrissmith.house.gov/uploadedfiles/2011.05.24_international_child_abduction_-_broken_laws_and_bereaved_lives.pdf
 2011-2013 Outgoing Cases,
 Annual Reports on International Child Abduction, 2015 (for CY2014), 2016 (for CY2015), 2017 (for CY2016), 2018 (for CY2017), 2019 (for CY2018), 2020 (for CY2019),
 December 10, 2018 House FA Subcommittee hearing transcript, "REVIEWING INTERNATIONAL CHILD ABDUCTION",
page 11, https://docs.house.gov/meetings/FA/FA16/20181210/108779/HHRG-115-FA16-Transcript-20181210.pdf
 From a published recording of a public seminar co-organized by the Ministry of Foreign Affairs of Japan and the Japan Federation of Bar Associations, "Caught! Japan Seminar On How To Abduct", http://www.bachome.org/news/caught-japan-seminar-on-how-to-abduct