A back plate is biased radially inward of a disc rotor by a spring, engaged at two locations on a torque reception surface with a caliper support shaft, and engaged at one location on a torque reception surface with a caliper support shaft. Inner and outer shims are disposed toward a back plate and pistons, respectively, and a friction factor between the back plate and the inner shim and that between the pistons and the outer shim are higher than that between the inner and outer shims. The inner shim is movable with the back plate in radial and circumferential directions and is movable relative to the back plate in the axial direction, and the outer shim is movable with the back plate in radial and circumferential directions, is movable relative to the back plate in the axial direction, and is rotatable about the radially inner support shaft axis.