To provide a noise-free and inexpensive and non-contact-type reverse rotation preventive device for a small-sized motor.

Rachets A and B, where teeth in the shape of saw teeth provided severally in the axial direction are arranged in an annular form, are formed at the opposite faces of the bearing 2, constituting a fixed member and the rotating plate 4 constituting a rotating member which are arranged facing in a pair via a lubricant on the same axis. The teeth of the rachet A, B are made into sharply sloped surfaces and into gently sloped surfaces to the opposite faces 2dd and 4bb, and further they are arranged so that the sharply sloped surfaces and the sharply sloped surfaces, and the gently sloped surfaces and the gently sloped surfaces respectively face each other.