"Royalties from the brain training software for the Nintendo DS alone have reached 2.4 billion yen ($US22 million), with 17 million titles sold worldwide since its debut in Japan in May 2005.
Under the rules of his employer, state-funded Tohoku University, Kawashima could take up to half the proceeds with the rest going to the school.
But Kawashima, married to a high-school classmate with four sons, is happy to live on his annual salary of around 11 million yen ($US100,000).
"To hear this may put you off - but my hobby is work," he told AFP in an interview at his office in the northern city of Sendai.
Asked whether he ever thought of taking the royalty money and moving to a tropical island, Kawashima simply said: "I wouldn't know what to do there. If I had such time to spare, I want to do my research.""