A caliper brake, comprising: a carrier in which a pair of pad plates (151, 152) are installed to move forward and backward toward a disk; a caliper housing slidably installed on the carrier and provided with a cylinder; a piston installed in the cylinder and configured to move forward and backward toward the pair of pad plates (151, 152) by a braking hydraulic pressure; a seal member in close contact with an outer surface of the piston and an inner surface of the cylinder, the seal member having a ring-shape; and a seal groove formed to be recessed in the inner surface of the cylinder to accommodate the seal member, the seal groove including a seating surface on which an outer circumferential surface of the seal member is seated, a front braking surface facing a front surface of the seal member, and a rear braking surface facing a rear surface of the seal member; wherein the front braking surface includes a protrusion protruding toward the seal member, and the seal groove comprises a front inclined surface inclined from the front braking surface in a forward direction of the piston; and a rear inclined surface inclined from the rear braking surface in a backward direction of the piston; the protrusion is formed in a middle of the front braking surface, and the protrusion is formed to be curved on the front braking surface, and an inner space of the seal groove is divided into a first space positioned in front of the seal member and provided between the seal member and the piston, a second space positioned in front of the seal member and provided between the seal member and the seating surface, and a third space positioned at a rear of the seal member, and a volume of the first space is greater than that of the second space, and the second space is filled before the first space by elastic deformation of the seal member when the piston moves forward, and a part of the front surface of the seal member is in close contact with the front braking surface of the seal groove, and the remaining part thereof moves toward the front inclined surface to fill the second space. a carrier in which a pair of pad plates (151, 152) are installed to move forward and backward toward a disk; a caliper housing slidably installed on the carrier and provided with a cylinder; a piston installed in the cylinder and configured to move forward and backward toward the pair of pad plates (151, 152) by a braking hydraulic pressure; a seal member in close contact with an outer surface of the piston and an inner surface of the cylinder, the seal member having a ring-shape; and a seal groove formed to be recessed in the inner surface of the cylinder to accommodate the seal member, the seal groove including a seating surface on which an outer circumferential surface of the seal member is seated, a front braking surface facing a front surface of the seal member, and a rear braking surface facing a rear surface of the seal member; wherein the front braking surface includes a protrusion protruding toward the seal member, and the seal groove comprises a front inclined surface inclined from the front braking surface in a forward direction of the piston; and a rear inclined surface inclined from the rear braking surface in a backward direction of the piston; the protrusion is formed in a middle of the front braking surface, and the protrusion is formed to be curved on the front braking surface, and an inner space of the seal groove is divided into a first space positioned in front of the seal member and provided between the seal member and the piston, a second space positioned in front of the seal member and provided between the seal member and the seating surface, and a third space positioned at a rear of the seal member, and a volume of the first space is greater than that of the second space, and the second space is filled before the first space by elastic deformation of the seal member when the piston moves forward, and a part of the front surface of the seal member is in close contact with the front braking surface of the seal groove, and the remaining part thereof moves toward the front inclined surface to fill the second space.