PURPOSE: To return a magnet supporting wheel to a nonbraking position by releasing the engagement between a small gear and the partial gear of the magnet supporting wheel at trouble of a motor, etc.

CONSTITUTION: Many ferromagnetic plates 31 are arranged at equal intervals in circumferential direction at the outer tube of a fixed frame 40 arranged inside a braking drum 32. A magnet 36 corresponding to each ferromagnetic plate 31 is coupled, so that the polarities may be different alternately in circumferential direction, with the magnet supporting wheel 37 supported rotatably inside the fixed frame 40. In connection with the disposition pitch of the ferromagnetic plates 31, the magnet supporting wheel 37 is rotated forward and reversely to the braking position and nonbraking position. A small gear 21 is engaged with the partial gear 22 coupled with the magnet supporting wheel 37. The shaft 20 of the small gear 21 is engaged by spline radially movably with the boss part 18a of the gear 18 of the worm gear mechanism 18 driven by a motor 10. This is equipped with an electromagnetic actuator b which biases the shaft 20 of the gear to the position where the small gear 21 engages with the partial gear 22, against the spring force.