To reduce the size of whole device by providing three screw clusters in triangle shape and two drive wheels and one guide ball at diagonal positions of each angle and neutralizing the buoyancy.

Inside the side surfaces 1a, 1b and 1c of a float body 1, three screw clusters 2a, 2b and 2c and arranged and at the angle parts 1d and 1f, 2 drive wheels 4a-1 and 4b-1 having steering mechanisms 4a-2 and 4b-2 and containing a rotational drive device and a guide ball 5 supported freely rotatably are arranged. A the angle 1e of the body 1, inclination meters 3a, 3b and 3c for three axis directions to detect the posture angle of an underwater inspection device I are provided. Also, in the center of the body 1, X Y axis distance meter 6 of track ball type measuring the running distance in X axis and Y axis directions of the inspection device I are provided. Furthermore, on the wall surface 15 side of the angles 1d, 1e and 1f, sucker mechanisms 7a, 7b and 7c are arranged.