PURPOSE: To miniaturize arms of a brake support by forming a groove and a projection in a complementary semicircular shape in respective guides for receiving brake force and transmitting the brake force to the brake support through the guides.

CONSTITUTION: A semicircular cross-sectional groove 54 is formed in parallel to the axis A in a guide body 50 having a piston 52, and a complementary semicircular projection 56 is guided in this groove 54. When brake force larger than usual is applied a contact state between the complementary semicircular groove 54 and the projection 56 does not change, so that guides 30, 32 are not abraded larger than abrasion in the case of an ordinary load. A threaded pin 60 screwed in arms 14, 16 hardly changes in a contact state, and can pivotally move along the guide body 50 in a range of a comparatively large angle B with a central point C of the projection 56 as the center. Thus, since the guides 30, 32 are not damaged even by abnormally large brake force, it is not necessary to form the arms 14, 16, 18, 20 for supporting the guides 30, 32 in an excessively storing dimension.