Abstract: A disk brake assembly includes a rotary brake disk rotatable with a wheel and a mounting member fixed to a stationary member of a vehicle. A guide member is fixed at one end portion to the mounting member. A caliper member having a tubular portion slidably supported on the guide member is movable in an axial direction with the caliper member straddling a portion of the brake disk and having a fluid actuator provided with a piston on one side thereof to press an inner brake pad against one face of the brake disk and a reaction portion on the opposite side thereof to press an outer brake pad against the other face of the brake disk. The outer brake pad has a projection portion projected outwardly of the brake disk at the center of its backing plate and a spring member is disposed between the outer brake pad and the reaction portion and rigidly supported the outer brake pad on the caliper member.