To provide a hydraulic vehicular disc brake with a piston seal having improved durability, for securing a proper roll-back amount and a stable feeling of operation.

The piston seal 11 and a piston seal mounting groove 9 are formed in such shapes that in the state of fitting the piston seal 11 into the piston seal mounting groove 9, the outer peripheral face 11a of the piston seal 11 and the bottom face 9a of the piston seal mounting groove 9 are closely contact with each other, the cylinder hole opening-side side face 11b of the piston seal 11 and the cylinder hole opening-side wall face 9b of the piston seal mounting groove 9 are closely contact with each other, and the angle 11c of the piston seal 11 and the corner 9f of the piston seal mounting groove 9 are closely contact with each other. The cylinder hole opening-side wall face 9b of the piston seal mounting groove 9 is inclined at its inner periphery side to the cylinder hole opening side to form a roll-back generation plane 9c apart from the cylinder hole opening-side side face of the piston seal.