To eliminate a torque change due to braking medium and allow effective braking moment to be caused by pushing a disc rotor having annular friction faces aligned in the axial direction against the friction surface of a housing in the axial direction by using a spring in the housing.

A housing 10 has a pot-like portion 12 with flat projections 14, 16 on both sides, the pot-like portion 12 being closed at its open side by a cover 22. A disc rotor 28 is arranged in a housing 10 and passed through a center hole in a cover 22 on the larger diameter side of am axial portion 30 molded on the disc rotor 28. The disc rotor 28 is supported at its shoulder 34 on the cover 22, which, at the opposite side, is engaged with the wall of the housing 10 and supported by a friction surface 36 fixed to the housing 10. A cup spring 36 is supported by the cover 22 on the outer periphery of the cup spring 36 in the radial direction while the radial inner periphery engages the disc rotor 28 to cause relatively low friction force during movement of the disc rotor 28.