A vehicle disc brake including: a caliper body having a pair of operation arms (3a, 3a) disposed to face both sides of a disc rotor and a bridge portion disposed to connect the operation arms (3a, 3a) and to straddle an outer circumference of the disc rotor, each operation arm having a cylinder bore that opens towards the side of the disc rotor and a pair of pad guide grooves (3i, 3i); pistons each formed with an opening portion and inserted into the corresponding cylinder bore such that the opening portion thereof is oriented towards the side of the disc rotor; a pair of friction pads (4, 4) disposed on the both sides of the disc rotor, each friction pad having a lining; a back plate that supports the lining; a pair of lug pieces (4a, 4a) that are provided at a turning disc rotor entrance side and a turning disc rotor exit side of the back plate, which are defined with reference to a vehicle forward traveling state, and that are accommodated in the pad guide grooves (3i, 3i) so that the friction pads (4, 4) are movable in an axial direction of the disc rotor, and a projection (4d, 4e, 4f) that projects from the back plate towards a bottom portion of the cylinder bore, characterized in that the projection (4d, 4e, 4f) projects from the back plate towards the bottom portion of the cylinder bore so as to be positioned inside the opening portion of the piston with a gap from an inner circumferential surface of the piston. a caliper body having a pair of operation arms (3a, 3a) disposed to face both sides of a disc rotor and a bridge portion disposed to connect the operation arms (3a, 3a) and to straddle an outer circumference of the disc rotor, each operation arm having a cylinder bore that opens towards the side of the disc rotor and a pair of pad guide grooves (3i, 3i); pistons each formed with an opening portion and inserted into the corresponding cylinder bore such that the opening portion thereof is oriented towards the side of the disc rotor; a pair of friction pads (4, 4) disposed on the both sides of the disc rotor, each friction pad having a lining; a back plate that supports the lining; a pair of lug pieces (4a, 4a) that are provided at a turning disc rotor entrance side and a turning disc rotor exit side of the back plate, which are defined with reference to a vehicle forward traveling state, and that are accommodated in the pad guide grooves (3i, 3i) so that the friction pads (4, 4) are movable in an axial direction of the disc rotor, and a projection (4d, 4e, 4f) that projects from the back plate towards a bottom portion of the cylinder bore, a lining; a back plate that supports the lining; a pair of lug pieces (4a, 4a) that are provided at a turning disc rotor entrance side and a turning disc rotor exit side of the back plate, which are defined with reference to a vehicle forward traveling state, and that are accommodated in the pad guide grooves (3i, 3i) so that the friction pads (4, 4) are movable in an axial direction of the disc rotor, and a projection (4d, 4e, 4f) that projects from the back plate towards a bottom portion of the cylinder bore, characterized in that the projection (4d, 4e, 4f) projects from the back plate towards the bottom portion of the cylinder bore so as to be positioned inside the opening portion of the piston with a gap from an inner circumferential surface of the piston.