Disk brake

A disk brake, comprising:

• a brake pad that is arranged on each side of a disk rotor;
• a piston that presses the brake pad to the disk rotor;
• a cylinder bore into which the piston is installed;
• an annular sealing groove that is formed on an inner periphery of the cylinder bore;
• and
• a piston seal that is rectangular in section and is fitted into the sealing groove so as to seal a portion defined between the piston and the cylinder bore,
• wherein the sealing groove is composed of:
• a bottom surface portion inclined as the sealing groove expands its diameter as moving toward an opening of the cylinder bore;
• a lateral surface portion that extends from one end of the bottom surface portion, the one end being a side where the sealing groove has the maximum diameter;
• and a chamfered portion that is provided between the lateral surface portion and the inner periphery of the cylinder bore, the chamfered portion being formed as the opening of the sealing groove is expanded in an axial direction of the cylinder bore, and
• wherein an inclined angle of the bottom surface portion relative to an axis of the cylinder bore is set to 2 to 5 degrees, an angle defined between the bottom surface portion and the lateral surface portion is set to 85 to 88 degrees, and in a condition where the piston is installed into the cylinder bore, the piston seal is adapted to abut against the bottom surface portion and the lateral surface portion of the sealing groove.