A disk brake comprising: a rotor; and a disk brake pad (18a, 18b) supported on a pad support member movably in an axial direction of the rotor, comprising: a lining; and a back plate supporting a back surface of the lining, wherein shapes of opposite sides of the back plate in a circumferential direction of the rotor are asymmetric with respect to the circumferential direction, a first side part of the back plate in the circumferential direction includes a guided portion configured to engage movably in the axial direction with a guiding portion formed in the pad support member, and the guided portion is configured to engage with the guiding portion (26a, 26b) so as to support a brake tangential force applied toward a second side part of the back plate which is opposite to the first side part in the circumferential direction in braking, and the back plate is configured to support a brake tangential force applied toward the first side part in braking at a portion of the back plate that exists more outward in the radial direction than the line of action of the brake tangential force applied toward the first side part, so that the back plate is configured to receive a moment pressing down the second side part inward in the radial direction in both of forward-run braking and backward-run braking operations, characterized in that the guided portion is disposed at a position more inward in a radial direction of the rotor than a line of action of a brake tangential force applied in braking, and the guided portion is a pin insertion portion for inserting a pin (26a, 26b) used as the guiding portion supported and fixed in the axial direction to the pad support member. a rotor; and a disk brake pad (18a, 18b) supported on a pad support member movably in an axial direction of the rotor, comprising: a lining; and a back plate supporting a back surface of the lining, a lining; and a back plate supporting a back surface of the lining, wherein shapes of opposite sides of the back plate in a circumferential direction of the rotor are asymmetric with respect to the circumferential direction, a first side part of the back plate in the circumferential direction includes a guided portion configured to engage movably in the axial direction with a guiding portion formed in the pad support member, and the guided portion is configured to engage with the guiding portion (26a, 26b) so as to support a brake tangential force applied toward a second side part of the back plate which is opposite to the first side part in the circumferential direction in braking, and the back plate is configured to support a brake tangential force applied toward the first side part in braking at a portion of the back plate that exists more outward in the radial direction than the line of action of the brake tangential force applied toward the first side part, so that the back plate is configured to receive a moment pressing down the second side part inward in the radial direction in both of forward-run braking and backward-run braking operations, characterized in that the guided portion is disposed at a position more inward in a radial direction of the rotor than a line of action of a brake tangential force applied in braking, and the guided portion is a pin insertion portion for inserting a pin (26a, 26b) used as the guiding portion supported and fixed in the axial direction to the pad support member.