To simplify a structure for energizing a driven shaft 31 toward a drive shaft 30.

A pair of spring pieces 8a having hook parts 38c and 38d are provided at the both ends of a coil part 38 and both hook parts 38d are formed into a single body. Thus, a pair of the spring pieces 38a are unified together, and a pair of the hook parts 38c are hooked onto the drive shaft 30 and the hook parts 38d are hooked onto the driven shaft 31.