A pad assembly for a disc brake, the pad assembly comprising: a pad in which a lining is fixedly attached to a front surface of a pressure plate, and which is to be arranged to face an axial-direction side surface of a rotor; and a shim plate having a back surface to be pressed by a leading end face of a piston, wherein the shim plate includes a flat plate-shaped main body and a locking piece (13d, e, f) folded from a peripheral edge of the main body toward the pressure plate, wherein the locking piece (13d, e, f) is engaged with a peripheral edge of the pressure plate in a state where a front surface of the main body is superimposed directly or through another shim plate on a back surface of the pressure plate, wherein the pressure plate includes, in a part of the back surface of the pressure plate which is shifted from a part of the pressure plate opposed to the leading end face of the piston, an engaging projection projecting in an axial direction of the rotor, wherein the engaging projection is inserted into an engaging hole formed in a part of the main body of the shim plate which is shifted from a part of the main body to be contacted with the leading end face of the piston in a state where a flat surface part of the engaging projection constituting a side surface of the engaging projection and a linear part of the engaging hole constituting an inner peripheral edge of the engaging hole are opposed to each other and where the flat surface part and the linear part are inclined to each other with the shim plate existing at a neutral position with respect to the pressure plate, wherein, of shifting movements of the shim plate relative to the pressure plate, a movement of the shim plate in a circumferential direction of the rotor is regulated by an engagement between the locking piece (13d, e) and the inner peripheral edge of the pressure plate, and wherein, of a rotation of the shim plate about an axis of the piston, a rotation in a direction to act on the piston at least in braking is regulated by a line contact between the flat surface part and the linear part. a pad in which a lining is fixedly attached to a front surface of a pressure plate, and which is to be arranged to face an axial-direction side surface of a rotor; and a shim plate having a back surface to be pressed by a leading end face of a piston, wherein the shim plate includes a flat plate-shaped main body and a locking piece (13d, e, f) folded from a peripheral edge of the main body toward the pressure plate, wherein the locking piece (13d, e, f) is engaged with a peripheral edge of the pressure plate in a state where a front surface of the main body is superimposed directly or through another shim plate on a back surface of the pressure plate, wherein the pressure plate includes, in a part of the back surface of the pressure plate which is shifted from a part of the pressure plate opposed to the leading end face of the piston, an engaging projection projecting in an axial direction of the rotor, wherein the engaging projection is inserted into an engaging hole formed in a part of the main body of the shim plate which is shifted from a part of the main body to be contacted with the leading end face of the piston in a state where a flat surface part of the engaging projection constituting a side surface of the engaging projection and a linear part of the engaging hole constituting an inner peripheral edge of the engaging hole are opposed to each other and where the flat surface part and the linear part are inclined to each other with the shim plate existing at a neutral position with respect to the pressure plate, wherein, of shifting movements of the shim plate relative to the pressure plate, a movement of the shim plate in a circumferential direction of the rotor is regulated by an engagement between the locking piece (13d, e) and the inner peripheral edge of the pressure plate, and wherein, of a rotation of the shim plate about an axis of the piston, a rotation in a direction to act on the piston at least in braking is regulated by a line contact between the flat surface part and the linear part.