Heat generation is not caused by friction in a phase varying apparatus for use with a vehicle engine.A phase varying apparatus for use with a vehicle engine varies the opening/closing timing of an intake valve or an exhaust valve. The phase varying apparatus includes a rotary drum (44A) screwed to an intermediate member (30); an electromagnetic clutch (42) including a plurality of magnets (80) that are fixedly arranged at predetermined intervals along the circumferential direction of the rotary drum and that are magnetized alternately in opposite directions and a coil wound around an iron core (60); and ferromagnetic-material-made magnetic flux induction members (82, 84) having a plurality of claws (82B, 84B) close to magnetic poles of a magnet, having a slight gap between the iron core and the magnetic flux induction member, and forming a magnetic path (85) made up of the iron core and the magnets. The magnetic flux induction member is fixed to an outer cylinder part (10) or to an inner cylinder part (20). The rotary drum is accelerated and decelerated by a magnetic force exerted on the magnet by the claws while controlling an electric current to be supplied to the coil, and the inner cylinder part (20) and the outer cylinder part connected to the intermediate member in a spline manner are rotated relative to each other.