Positioning method for underwater moving robot

A supersonic flaw detection unit which generates supersonic waves and receives reflection waves is attached to a robot main body, the robot main body is moved along wall surfaces, and supersonic waves are generated from the supersonic flaw detection unit to the inside of the wall surface of a pressure vessel. The kind of the protrusion formed on the outer wall surface of a pressure vessel and the position of the robot main body relative to the protrusion is detected and then positioning is conducted after determining the position of the robot main body relative to the inner wall surface. The kinds and positions of mounted protrusions such as nozzles and brackets are previously confirmed by designed dimension or drawings, accordingly, if the kind and the position of the protrusion are detected, the positioning of the starting point of the robot relative to the pressure vessel can be performed easily at a high accuracy. (N.H.)