The discharge pressure value is made to smoothly approach the predetermined pressure value and once the discharge pressure value reaches the predetermined pressure value, the fluctuation of the discharge pressure value is decreased. In case the difference C between the discharge pressure value P(B) of the pneumatic regulator and the predetermined pressure value A is relatively large, the solenoid valve 31 is driven by the pulse-width modulation signal D and hence, the rapidity of the input and output response (the variation characteristics of the pressure detection signal B relative to the predetermined pressure signal A) is held, while in case the discharge pressure value P is close to the predetermined pressure value Q, the solenoid valve 31 is driven by the pulse frequency modulation signal J and hence, the amount of excess, a so-called overshoot can be decreased and even after the discharge pressure value P reaches the predetermined pressure value Q, the fluctuation of the discharge pressure value P, namely, the ripple can be decreased.