[3][4] Seo met Kenzō Masaoka and joined his company, working on Japan's first sound animation film, Chikara to Onna no Yo no Naka, before starting his own production company in 1935, where he made cartoons featuring the character Norakuro.
[3] He joined the Geijutsu Eigasha studio in 1937[5] and made Ari-chan in 1941, the first Japanese work to fully use the multiplane camera.
(Momotarō no Umiwashi was advertised at the time as the first feature-length anime, but since it is only 37 minutes long, today most recognize the 74-minute Umi no Shinpei as the first.
[8] After the war, Seo joined Nihon Manga Eigasha and made the film Ōsama no Shippo as a pro-democracy anime in 1949, but when Tōhō, which was supposed to distribute it, found it politically too leftist, the film was left without a distributor.
[9] Nihon Manga Eigasha went bankrupt, and Seo, finding the conditions for animation in the immediate postwar too difficult, left the industry and became an illustrator for children's books.