A drive device is provided with a housing 11 and an electromagnetic clutch mechanism 14 accommodated in the housing. The electromagnetic clutch mechanism 14 is provided with an armature 41 rotatably supported in the housing 11, a rotor 42 arranged to face the armature 41 and rotatably supported in the housing 11, and a coil 46. An annular magnet 44 is fixed on the outer circumference surface of the rotor 42. A sensor 15 arranged inside the housing 11 detects a polarity change of the magnet 44 in association with rotation of the rotor 42. The magnet 44 is provided with an annular wall portion 44b extending toward the armature 41 beyond a surface on which the armature 41 is in contact with the rotor 42 with respect to the axial direction. The annular wall portion 44b is spaced radially outward from the armature 41. The annular wall portion 44b prevents abrasion powder produced on actuation of the electromagnetic clutch mechanism 14 from being scattered.