PURPOSE: To exhibit a large holding force with a small current by feeding a current to a plurality of coils while switching sequentially and exciting the end face of pole sequentially in the circumferential direction thereby shifting the relative position of a point at which a force bearing member is attracted to the pole.

CONSTITUTION: When a current is fed between brushes R and CO, poles A and B are excited and the pole B is attracted to an iron plate 3 whereas the pole A does not slip but rolls to the right due to magnetic friction force because the pole A touches the iron plate 3. When the commutator is switched to excite the poles B and C, the pole C is attracted to the iron plate 3 whereas the pole B rolls to the right while touching the iron plate due to the magnetic friction force. When a current is fed between brushes L and CO, the poles A and Z are excited and the pole Z is attracted to the iron plate 3 whereas the pole A rolls to the left while the iron plate due to the magnetic friction force. When a current is fed between the brushes R and L, the poles B and Z are excited and they do not roll but are blocked. When the current supply is interrupted, the poles B and Z are unlocked and freed.