Japan actually had a quite large socialist/communist movement in the early 20th century. You just have to find a POD which results in Japan losing the Russo-Japanese War and they get the same revolutionary energy as IOTL the Bolsheviks.
This is not ASB. "ASB" and "impossible" are thrown around way too much here. "Unlikely" would be a better word for it, but definitely not outright impossible.
Alright, so what you would probably need for After 1900 is for Japan to lose the Russo-Japanese War. So let's say that the Russians aren't completed defeated early on and their manpower advantage becomes too much for Japan to handle. This could lead to the people deciding not to trust the Imperial Family and a socialist movement grows despite repression from the government, which could lead to events similar to Russia in OTL. If the Russian Revolution and a Russian Civil War where the Reds win still happens, that would also be good for a growing communist movement. Maybe the breaking point comes if Japan fares particularly poorly during WW1, and many people feel that Japan has been cheated out of their possessions by the British and the French, leading to anti-Western anger. A civil war erupts in around the 20s, resulting in a communist victory assisted by the Soviets. Korea and Taiwan are probably taken alongside it. There actually was a socialist party in Japan founded in 1906 called the Japan Socialist Party, but it dissolved pretty quickly since there was quite a bit of infighting and it was banned by the Japanese government in 1907. Given some circumstances such as Japan losing the Russo-Japanese War, maybe this party becomes more popular and stays united.