"Training at the Russian Cosmonaut Training Center begins today," Maezawa tweeted.
Last month, the Space Adventures company said that Maezawa and his assistant Yozo Hirano would fly to the ISS aboard Russian spacecraft Soyuz MS-20 as tourists. The launch is planned to be from the Baikonur cosmodrome on 8 December.
Russia's state space agency Roscosmos and Space Adventures have been cooperating on space tourism since 2001, when the first space tourist, US millionaire Dennis Tito, went into orbit. A total of seven people have been since sent to the ISS under the space tourism program.