Quoting from https://en.wikipedia.org/wiki/Energy_in_Japan#National_grid https://en.wikipedia.org/wiki/Energy_in_Japan#National_grid "The two grids were origin

Quoting from https://en.wikipedia.org/wiki/Energy_in_Japan#National_grid https://en.wikipedia.org/wiki/Energy_in_Japan#National_grid "The two grids were originally developed by separate companies. [..] In 1885 demand had grown enough that TELCO bought generation equipment from AEG of Germany. The same happened in the western parts of Japan with General Electric being the supplier to Osaka Electric Lamp. GE's equipment used the US standard 60 Hz while AEG's equipment used the European standard of 50 Hz." AFAIK they never managed to convert the network to a common frequency due to cost concerns and politics.