an operating lever 9 and a return spring not shown is fitted into the square shaft 6a into the center of the male screw member 6 is fitted a manual adjustment bolt 11 from the outside the adjustment bolt 11 is provided with a lock unit 12 when the lock nut 12 is fastened to both the square shaft 6a and the brake operating lever 10 via a washer 13 the brake operating lever 10 the square shaft 6a and the adjustment bolt 11 can be secured to one another the automatic gap adjustment device a of this invention which is to be interposed between the male screw member 6 and the movable friction pad 3 inside the abovementioned mechanism chamber 5 is constructed in the following manner namely the automatic gap adjustment device a consists of a pressure bearing plate 14 fixed to a back plate 3a of the movable friction pad 3 an adjustment screw 15 having its one end screw engaged with a screw hole at the center of the pressure bearing member 14 and the other end slidably and rotatably supported by the center hole of the male screw member 6 a ratchet wheel 16 secured at the.