Abstract: A submarine boat of the engineless type wherein the propulsion and vertical displacement thereof are distinctively obtained by cooperatively changing the longitudinal angle of inclination or pitch attitude and the buoyancy thereof. This submarine boat includes an elongated shell having apertures allowing water in and out thereof, an air-tight cabin, a pair of bellows extending lengthwise longitudinally of the shell and each having an open end communicating with the cabin, hydraulic cylinders extending into the bellows and connected to the other end of the latter respectively, and separate two-way valves connected to the hydraulic cylinders respectively to selectively actuate the latter, thereby producing selective expansion or contraction of the bellows.