Abstract: It is an object to obtain an air disc brake in which reduction in size, in weight, and in cost can be attained, and the responsibility can be improved easily. A pair of leg portions 15a in opposition to each other so as to put therebetween a rotor 3 are provided in a caliper body 15 fixed to a truck frame 7, a pair of brake blocks 14 disposed in opposition to each other so as to put therebetween the rotor 3 are attached to air chambers 17 built in the leg portions 15a respectively, each of the air chambers 17 includes a diaphragm 27 which is displacable to the rotor 3 side by a supplied air pressure, a piston 29 having a base end in contact with the diaphragm 27 and a tip end connected to the brake block 14, and a return spring 31 for urging the piston 29 to move apart from the rotor 3.