To transmit the operation of a parking brake to a piston by forming the inner diameter of a hole of a body for slidably supporting a tappet smaller than that of a body for slidably supporting the piston.

A pair of tappets 19 having an outer end surface received the inner end surface of a piston 3 are slidably arranged between pistons 3 opposed to a regulator in an axial direction in the hole 2' of a body 1. When a parking brake 20 is operated, the tip end both side surfaces of a wedge 22 moved vertically are opposed to the inner end surface of the tappet 19 through a roller 23, the piston 3 is pushed outward the axial direction by descent of the wedge 22, and brake is applied. The inner diameter of the hole 2' is formed smaller than that of the hole 2 in which the piston 3 is moved, both parts are connected to each other on an inclining surface 24, an intermediate member 21 having an inclining surface which collides with the inclining surface 24 is interposed between inner end surfaces of the tappet 19 and the piston 3, and it is obstructed that the piston 3 is moved inward the axial direction. It is thus possible to integratedly assemble both brakes for oil and parking with each other with a simple structure.