There are two MSCA calls available for Japanese researchers (Japan-based), MSCA-ITN (Innovative Training Networks) or MSCA-IF (European Fellowship). For MSCA-ITN you will need to monitor open positions, for example through EURAXESS or a Japanese University might be participating. For the European Fellowship, please note that you will need to make your submission through a European institute and that you will need to spend the whole period in Europe. Please check the guidelines for requirements and procedures on the Participant Portal.
(The third main MSCA is RISE and is not open for individual application.)