In a power transmitting device, projections are formed on a surface of a main cam that faces an armature. This structure causes the main cam to contact the armature only at distal surfaces of the projections. The area of the distal surfaces of the projections is in a range where the magnetic flux density generated at the distal surfaces of the projections by the electromagnet is saturated. Therefore, reduction of the responsiveness and controllability of the power transmitting device due to magnetization of the main cam is suppressed.