In the 60s he enters Toei Doga (the most important animation studio these days) as animator. Here he formed a team with Yasuo Ôtsuka, Isao Takahata and Hayao Miyazaki and worked together in the animation films "Prince of the Sun: The Great Adventure of Horus" (1968) and "The Mysterious Flying Boat" (1969). In 1968 he is promoted to animation director with the TV series "Magical Sally". In 1972 all the team quits Toei to work within Tokyo Movie Studio, and here Kotabe does the animation direction and/or character design of "Panda Kopanda" and its sequel "Panda Kopanda Rainy Circus" (1973). Later, as a free-lance artist he does the animation direction and character design of the TV series and film "Heidi, the Little Girl of the Alps" (1974), his very first hit, and "Marco, 3000 Leagues in Search of Mother" (1976). After a decade of glorious collaborations with Takahata, he joined Nintendo Video Game Company in 1985 as a creative designer and begins to work in the "Pocket Monsters" saga and later in the animation TV series and film conversions.