Abstract: A semisubmersible system for collecting a desired material from the sea water includes a submersible collection chamber which is defined by a pair of non-porous, buoyant side walls, a pair of non-porous end walls, a porous top wall and a porous bottom wall. A quantity of collecting agent is contained in the collection chamber. The buoyancy force applied to the present system by the pair of buoyant side walls is so set to keep the collection chamber substantially at a predetermined depth from the sea surface, so that the sea water is permitted to move into and out of the collection chamber due to wave motion while keeping the collection chamber virtually stationary in the vertical direction. There is obtained a large relative motion between the sea water and the collection chamber so that a large quantity of sea water is brought into contact with the collecting agent, thereby allowing a useful material contained in the sea water to be collected by the collecting agent by adsorption or absorption.