A disc brake pad assembly comprising: a pad which includes a lining additionally attached to a front surface of a pressure plate (2, 29) and which is disposed at a portion facing an axial side surface of a rotor; an inner shim plate (3, 3a) which has a flat plate-shaped inner main body portion (7, 7a) additionally provided on a back surface of the pressure plate included in the pad; an outer shim plate (4, 4a) which has a flat plate-shaped outer main body portion (12, 12a) superposed on the inner main body portion (7, 7a) of the inner shim plate (3, 3a), and a pair of locking bent portions which are formed at circumferential end portions of the inner shim plate (3, 3a) by being bent from the inner main body portion (7, 7a) towards an opposite side to the pressure plate (2, 2a); characterized in that locking through holes which are formed individually in widthwise central portions at base end portions of the locking bent portions; and a pair of locking projecting pieces which are individually formed so as to project circumferentially from radial middle portions of circumferential end edges of the outer main body portion (12, 12a) included in the outer shim plate (4, 4a), wherein a distance between distal edges of the locking projecting pieces is larger than a space between the locking bent portions, and a circumferential length defined between radial side portions of the locking projecting pieces on the circumferential end edges of the outer main body portion (12, 12a) is shorter than the space between the locking bent portions, and wherein the inner main body portion (7, 7a) and the outer main body portion (12, 12a) are superposed on each other in a state that the locking projecting pieces are brought into engagement with the locking through holes. a pad which includes a lining additionally attached to a front surface of a pressure plate (2, 29) and which is disposed at a portion facing an axial side surface of a rotor; an inner shim plate (3, 3a) which has a flat plate-shaped inner main body portion (7, 7a) additionally provided on a back surface of the pressure plate included in the pad; an outer shim plate (4, 4a) which has a flat plate-shaped outer main body portion (12, 12a) superposed on the inner main body portion (7, 7a) of the inner shim plate (3, 3a), and a pair of locking bent portions which are formed at circumferential end portions of the inner shim plate (3, 3a) by being bent from the inner main body portion (7, 7a) towards an opposite side to the pressure plate (2, 2a); characterized in that locking through holes which are formed individually in widthwise central portions at base end portions of the locking bent portions; and a pair of locking projecting pieces which are individually formed so as to project circumferentially from radial middle portions of circumferential end edges of the outer main body portion (12, 12a) included in the outer shim plate (4, 4a), wherein a distance between distal edges of the locking projecting pieces is larger than a space between the locking bent portions, and a circumferential length defined between radial side portions of the locking projecting pieces on the circumferential end edges of the outer main body portion (12, 12a) is shorter than the space between the locking bent portions, and wherein the inner main body portion (7, 7a) and the outer main body portion (12, 12a) are superposed on each other in a state that the locking projecting pieces are brought into engagement with the locking through holes.