A shim plate is provided at the rear side of a friction pad to abut annular contact parts of pistons. The shim plate is provided with first and second one-side cut portions and at a position corresponding to the annular contact part of a disk rotational entrance-side piston, for example, so that the area of contact between the shim plate and the annular contact part is larger at the disk rotational exit side than at the disk rotational entrance side. The first and second one-side cut portions and have four contact ends at which the peripheries of the cut portions and the annular contact part intersect each other. At least two of the four contact ends are provided on the disk rotational entrance side with respect to the diametrical center of the piston.