The invention prevents overshoot from occurring in flow rate on the output side of a flow rate regulator when output flow rate is changed or the gas type distributed is changed. Thus, an automatic pressure regulator is provided to supply gas pressure to a flow rate regulator that includes a piezoelectric element driving type pressure regulating valve, a control pressure detector provided on the output side of the pressure regulating valve, and a controller to which a detected value P2 of the control pressure detector and a set value Pst for control pressure are input, wherein the controller supplies a control signal to a piezoelectric element driving unit of the pressure regulating valve using a proportional control system to perform valve opening regulation in which the proportional control system of the controller is set to control to bring about a residual deviation in control pressure by disabling an integral action.