A disk brake for a vehicle, comprising: a disk rotor a caliper bracket to be secured to a vehicle body, the caliper bracket having a pair of caliper support arms striding an outer edge of the disk rotor in a disk rotor shaft direction, the pair of caliper support arms extending along the caliper bracket, the pair of caliper support arms having U-shaped pad guide grooves formed by an outside surface in a radial direction of the disk rotor, an inside surface in the radial direction of the disk rotor, and a facing surface connecting the outside surface and the inside surface, the U-shaped pad guide grooves being provided so as to face each other; a pad retainer; and a pair of friction pads disposed so as to sandwich the disk rotor, the pair of friction pads each having a lug piece projecting from both sides of a back plate of each of the pair of friction pads, the lug piece being supported in the pad guide groove via the pad retainer, the pad retainer including a pair of retainer parts mounted in the pad guide grooves on both sides of the disk rotor, a coupling piece connecting the pair of retainer parts while striding the outer edge of the disk rotor, and a pad rebounding section biasing the lug piece toward the outside in the radial direction of the disk rotor, wherein the retainer parts each include an outside section disposed along the outside surface in the radial direction of the disk of the pad guide groove, a back section disposed along the facing surface, and an inside section provided so as to be inclined from the inside end of the back section in the radial direction of the disk rotor toward the outside in the radial direction of the disk rotor, the pad rebounding section is formed by inclining a leading end of a stripe extending toward the disk rotor via an elastic loop section bent like an arc toward the outside in the radial direction of the disk rotor, the elastic loop section extending from an end of the inside section away from the disk rotor, toward the outside in the radial direction of the disk rotor, the lug piece is biased by the pad rebounding section toward the outside in the radial direction of the disk rotor during travel of a vehicle, the lug piece being disposed in the pad guide groove with a gap provided between the inside section and the pad rebounding section, characterised in that, when the lug piece moves toward the inside in the radial direction of the disk rotor, the pad rebounding section comes into contact with the inside section, the inside section is pressed against the inside surface in the radial direction of the disk rotor, the back section is elastically deformed so as to warp toward the lug piece, and the lug piece is pressed toward a center of the friction pad by the elastically deformed back section. a disk rotor a caliper bracket to be secured to a vehicle body, the caliper bracket having a pair of caliper support arms striding an outer edge of the disk rotor in a disk rotor shaft direction, the pair of caliper support arms extending along the caliper bracket, the pair of caliper support arms having U-shaped pad guide grooves formed by an outside surface in a radial direction of the disk rotor, an inside surface in the radial direction of the disk rotor, and a facing surface connecting the outside surface and the inside surface, the U-shaped pad guide grooves being provided so as to face each other; a pad retainer; and a pair of friction pads disposed so as to sandwich the disk rotor, the pair of friction pads each having a lug piece projecting from both sides of a back plate of each of the pair of friction pads, the lug piece being supported in the pad guide groove via the pad retainer, the pad retainer including a pair of retainer parts mounted in the pad guide grooves on both sides of the disk rotor, a coupling piece connecting the pair of retainer parts while striding the outer edge of the disk rotor, and a pad rebounding section biasing the lug piece toward the outside in the radial direction of the disk rotor, wherein the retainer parts each include an outside section disposed along the outside surface in the radial direction of the disk of the pad guide groove, a back section disposed along the facing surface, and an inside section provided so as to be inclined from the inside end of the back section in the radial direction of the disk rotor toward the outside in the radial direction of the disk rotor, the pad rebounding section is formed by inclining a leading end of a stripe extending toward the disk rotor via an elastic loop section bent like an arc toward the outside in the radial direction of the disk rotor, the elastic loop section extending from an end of the inside section away from the disk rotor, toward the outside in the radial direction of the disk rotor, the lug piece is biased by the pad rebounding section toward the outside in the radial direction of the disk rotor during travel of a vehicle, the lug piece being disposed in the pad guide groove with a gap provided between the inside section and the pad rebounding section, characterised in that, when the lug piece moves toward the inside in the radial direction of the disk rotor, the pad rebounding section comes into contact with the inside section, the inside section is pressed against the inside surface in the radial direction of the disk rotor, the back section is elastically deformed so as to warp toward the lug piece, and the lug piece is pressed toward a center of the friction pad by the elastically deformed back section.