To prevent the bulging-out of a gasket by setting the contact surfaces of a brake piston and a guide block to be symmetrical left and right, and machining a groove adjacent to a gasket groove.

Gaps are formed in the vicinity of the outer peripheries of both end covers 9 such that a propelling force applied in the axial direction of a brake piston 3 is prevented from reducing the rotational moment of another brake piston 3. If a rotational moment is applied to the brake piston 3, the contact surface of the brake piston 3 is deformed (elastic deformation, plastic deformation or deformation by friction) by a resisting force from a guide block 1. Based on this deformation the rotational center of the inclination of the brake piston 3 is decided. Thus, by setting the contact surfaces of the brake piston 3 and the guide block 1 to be symmetrical left and right and bringing the pressure discharge side of the gasket groove of a gasket 5 to the centers of the contact surfaces, gaps where the gasket 5 does not bulge out are formed.