PURPOSE: To easily remove and recover algae without changing water quality of supernatant liquid by using a robot main body as a buoyancy body, sucking water of the outside from the suction plate of a side wall and moving the buoyancy body.

CONSTITUTION: The main body 5 of a cleaning robot is formed into a box body capable of floating on the water surface of a sedimentation basin. Furthermore, suction ports 12 are provided respectively on four faces. Algae stuck to the overflow notch parts 2a of supernatant liquid are sucked, removed and recovered by using a pump. A crawler type chain is detached from engagement of the overflow notch parts 2a at a time for changing the direction. A hollow bend arm integrated with an endless chain 10 is turned at 90° and engaged with the unevennesses of the following weir. Thereby, algae are easily removed and recovered by remote operation. Algae are removed without changing water quality of supernatant liquid.