PURPOSE : To simplify and miniaturize a structure, by a method wherein a coiled spring of small diameter and that of large diameter are arranged on a rotor shaft, each one end of the springs is connected and other end of the coiled spring of large diameter is fixed to the shaft bearing side of the rotor shaft. CONSTITUTION : A clutch spring 8 is arranged on the rotor shaft 7 of a permanent magnet rotor. The clutch spring 8 consists of a coiled spring 8a of small diameter and a coiled spring 8b of large diameter which are tightly wound. One end of the coiled spring 8b of large diameter is connected to one end of the coiled spring 8a of small diameter, and the other end of the spring 8b is fixed to a shaft bearing unit 6 with a nut 9. When the rotor shaft 7 is rotated in a direction A, it is rotatable because the tight portion 8a of the clutch spring 8 is uncoiled. The rotor shaft 7 is prevented from being rotated in B direction by the close contact portion 8a.