Patent ID: 8408366

Claim:
A disk brake comprising: brake pads disposed at sides opposite to each other across a disk rotor; a piston pressing the brake pad at least one side of the disk rotor against the disk rotor; a cylinder bore in which the piston is slidably fitted; a seal groove provided as an annular groove on an inner peripheral surface of a cylinder portion defining the cylinder bore; and a seal member having a rectangular sectional configuration, the seal member being fitted in the seal groove to seal between the piston and the cylinder bore; the seal groove having a bottom surface having: a first slant portion extending and increasing in diameter from a cylinder-opening-side seal groove wall surface of the seal groove, which is closer to an opening of the cylinder bore, toward a bottom of the cylinder bore in an axial direction of the cylinder bore; and a second slant portion provided closer to the bottom of the cylinder bore than the first slant portion and decreasing in diameter from the first slant portion toward the bottom of the cylinder bore in the axial direction; wherein a transition point between the first slant portion and the second slant portion is located closer to the opening of the cylinder bore than a center of the seal groove in the axial direction of the cylinder bore; and wherein, when no pressure is applied to the piston fitted in the cylinder bore, a gap is formed between an outer peripheral surface of the seal member and the bottom surface of the seal groove, and a surface pressure applied to the bottom surface of the seal groove from the seal member at a cylinder-bottom-side end of the seal member, which is closer to the bottom of the cylinder bore, by abutment of the seal member against the second slant portion is higher than a surface pressure applied to the bottom surface of the seal groove from the seal member at a cylinder-opening-side end of the seal member, which is closer to the opening of the cylinder bore, by abutment of the seal member against the first slant portion.