A shim assembly for a disk brake comprising: an inside shim plate configured to be mounted onto a back surface of a pressure plate of a pad for a disk brake, and including a flat plate-shaped inside shim main body; an outside shim plate including a flat plate-shaped outside shim main body overlapping with the inside shim main body of the inside shim plate; a plurality of inside shim locking pieces (8g, 8h, 8i, 8j), provided in a peripheral edge of the inside shim main body of the inside shim plate, bent toward the pressure plate and configured to be elastically contacted with the peripheral edge of the pressure plate, outside shim locking pieces (13g, 13h, 13i), provided in a peripheral edge of the outside shim main body of the outside shim plate, bent toward the pressure plate and configured to be contacted with or adjacently oppose to portions of the peripheral edge of the pressure plate, which are positioned out of portions thereof where the inside shim locking pieces contact, so that the outside shim plate is overlapping with a back surface of the inside shim plate characterized by engaging pawls (22a, 22b, 22c), provided in portions of the peripheral edge of the inside shim main body which are positioned out of portions thereof where the inside shim locking pieces (8g. 8h, 8i, 8j) are formed, each of which having a flat plate shape as a whole or having a tip end bent toward the pressure plate, and configured to project from the peripheral edge of the pressure plate in a state that the inside shim plate is mounted on the pressure plate, wherein the outside shim locking pieces (13g, 13h, 13i) are respectively engaged with the engaging pawls (22a, 22b, 22c) so as to cover the engaging pawls (22a, 22b, 22c), to prevent the outside shim plate against separation from the inside shim plate. an inside shim plate configured to be mounted onto a back surface of a pressure plate of a pad for a disk brake, and including a flat plate-shaped inside shim main body; an outside shim plate including a flat plate-shaped outside shim main body overlapping with the inside shim main body of the inside shim plate; a plurality of inside shim locking pieces (8g, 8h, 8i, 8j), provided in a peripheral edge of the inside shim main body of the inside shim plate, bent toward the pressure plate and configured to be elastically contacted with the peripheral edge of the pressure plate, outside shim locking pieces (13g, 13h, 13i), provided in a peripheral edge of the outside shim main body of the outside shim plate, bent toward the pressure plate and configured to be contacted with or adjacently oppose to portions of the peripheral edge of the pressure plate, which are positioned out of portions thereof where the inside shim locking pieces contact, so that the outside shim plate is overlapping with a back surface of the inside shim plate characterized by engaging pawls (22a, 22b, 22c), provided in portions of the peripheral edge of the inside shim main body which are positioned out of portions thereof where the inside shim locking pieces (8g. 8h, 8i, 8j) are formed, each of which having a flat plate shape as a whole or having a tip end bent toward the pressure plate, and configured to project from the peripheral edge of the pressure plate in a state that the inside shim plate is mounted on the pressure plate, wherein the outside shim locking pieces (13g, 13h, 13i) are respectively engaged with the engaging pawls (22a, 22b, 22c) so as to cover the engaging pawls (22a, 22b, 22c), to prevent the outside shim plate against separation from the inside shim plate.