PURPOSE: To prevent the deviation of a die height by a method wherein a diaphragm is operated by fluid supplied by a fluid supplying and discharging means to press first and second friction plates against each other to brake a rotary shaft.

CONSTITUTION: When a changeover valve 18 is controlled by a controller 19 so as to be positioned at a position A, air is supplied into an air chamber 12 from an air supplying and discharging port 14 through an air pressure source 16, a pressure reducing valve 17, a changeover valve 18 and an air supplying and discharging pipe 13. As a result, a diaphragm 11 is filled with air and is inflated and a friction plate 10 is moved toward a friction plate 6 together with a plug 8 to a position whereat the friction plate 10 is abutted against the friction plate 6. Accordingly, a rotary piece 5 and a rotary shaft 4 are braked by a pressing force between the friction plates 10, 6 due to the air pressure. Further, when a set pressure increasing signal is given to the actuator of a pressure reducing valve 17 by the controller 19, the set pressure of the pressure reducing valve 17 is increased. Accordingly, a pressure supplied into the air chamber 12 is increased and the pressing force between the friction plates 10, 6 is increased. According to this method, the rotary shaft 4 may be braked strongly further whereby the deviation of a die height may be prevented.