A semi-submersible displacement-type high-speed ship with wings is studied on its stability control by flaps. The vehicle, called HSV (High-speed Semi-submersible Vehicle with wings) here, is consisting of a main hull, main and tail wings with flaps, and a strut to support the super-structure. The HSV floats at a certain draft like a conventional ship while it is still, and it submerges when it moves forward due to a downward lift force induced by the main wings. HSV cannot have enough upright moment as conventional ships because of its small waterplane area. HSV is expected to be stable by an active control of flaps. Experiments are carried out to study about the possibility of the stability control by the flaps of the main and tail wings. An automatic control system is developed under which HSV is towed with the heaving, rolling, pitching and yawing motions free. The angles of attack of flaps are controlled after the measured ship motion in order to stabilize the motion. It is concluded that, although the control is not perfect due to the mechanical limitation of the used system, HSV can run stably by the active control of the flaps. The experiments among regular waves made clear that waves could not be a serious factor for the stability of HSV.