A desired search area is efficiently monitored. A surveillance device that is installed in a flying aircraft in the sky and that monitors the situation at the sea surface from the sky, including a light-transmitting portion that is provided with a light source and that radiates a plurality of slit-like beams from the sky toward a search area at the sea surface, a light-receiving portion that receives a plurality of reflected beams that have been reflected at an object at the sea surface, and a processor that determines whether or not the object at the sea surface that has reflected the beams is a target object from information about the reflected beams obtained by the light-receiving portion.