Katsuhiko Sasaki as Inventor Goro Ibuki

Yutaka Hayashi as Hiroshi Jinkawa

Hiroyuki Kawase as Rokuro 'Roku-chan' Ibuki

Kanta Mori as Japan Special Defense Forces Chief

Kotaro Tomita as Lead Seatopian Agent

Ulf Otsuki as Seatopian Agent

Sakyo Mikami as Truck Driver's Assistant

Fumiyo Ikeda as Man from Unit 1

Robert Dunham as Emperor Antonio of Seatopia

Gentaro Nakajima as Truck Driver

Ralph Jesser as Antonio's Aide (Radio Operator in White)

Shinji Takagi as Godzilla

Hideto Odachi as Megalon

Tsugutoshi Komada as Jet Jaguar

Masachika Mori as Jet Jaguar

Kenpachirō Satsuma as Gigan (as Kengo Nakayama)

Gorō Naya as Emperor Atonio of Seatopia (voice)

Eisuke Nakanishi as Narrator

Haruo Nakajima as Godzilla (archive footage) (uncredited)