Provided is an underwater posture stabilization system which, by being attached to various underwater equipment such as undersea equipment and underwater vehicles having different shapes and positions of the center of gravity according to their purpose and the like, allows simply and reliably setting their posture in water (basic posture), is excellent in versatility, is not only excellent in posture stability since, when the posture inclines under the effect and the like of tidal currents and pulsating currents, a moment to restore the posture to its original posture naturally acts, but is also capable of easily changing the posture as needed, and also is capable of stably controlling the posture even in the presence of complicated tidal currents and pulsating currents, and is excellent in reliability, stability, and certainty of posture control. The underwater posture stabilization system includes a frame to be attached to an apparatus body of the underwater equipment, a pair of sliding portions arranged so as to be freely movable back and forth on both side portions of the frame, a support member freely movable up and down which is fixed at a lower end thereof to each sliding portion, and arranged upward or obliquely upward, and a floating body arranged at a tip of each support member.