The INMARSAT service is a communication service that allows you to use phone and Internet through the INMARSAT geostationary satellites. This service makes communication possible on ships, land, deep in mountains with no communication setup, and disaster areas.
If you want to use INMARSAT for communications in Japan (including on-board vessels and aircraft registered in Japan), KDDI must first obtain a license for a wireless station from the Minister of Internal Affairs and Communications in accordance with the Radio Act in Japan, and then you enter into an agreement with KDDI to use the facilities.
Even if you have a license for your INMARSAT equipment for use in a country outside Japan, using it in Japan without a Japanese license is against the Radio Act.
For further information about the necessity of a wireless station license, please see the Ministry of Internal Affairs and Communications home page.