PURPOSE: To reduce a weight and an installation space by mounting a stopper rod on a caliper in opposition to the front of a brake piston, and determining a distance from the front of the brake piston to the stopper rod to a specific value.

CONSTITUTION: A stopper rod 5 is mounted on a caliper 1 in opposition to the front of a brake piston 2. A distance l from the front of the brake piston 2 which is at the start end of travel to the stopper rod 5, is determined to a value smaller than the maximum possible stroke L of the piston 2. The piston 2 hits the stopper rod 5 at the position where stroke of the distance, 1, is completed, and further advance of the piston 2 is prevented, thus preventing coming- off of the piston 2 from the outer peripheral seal section 3. In this way, it is not necessary to increase the dimension in axial direction of the piston, and it becomes advantageous with respect to its weight, installation space, and cost.