depth of the bottom 5 a of the accommodation portion 5 is determined based on the shape of the drive device 2 as shown in fig1 and 3 the drive device 2 includes a motor 2 a which is a drive source an output portion 2 b which has a gear attached to the motor 2 a and an electromagnetic clutch a control circuit portion 2 c which is installed in the output portion 2 b the accommodation portion 5 includes a motor accommodating section 5 b an output portion accommodating section 5 c and a circuit accommodating section 5 d as shown in fig4 a and 4b the drive device 2 is fixed by screws such that it partly contacts the bottom 5 a of the accommodation portion 5 that is a part of the drive device 2 is located lower than the upper surface 4 a of the flat plate portion 4 one third or more of the entire thickness of the drive device 2 in the vertical direction is located below the upper surface 4 a of the flat plate portion 4 in the present embodiment substantially half the entire thickness is located below the upper surface 4 a.