The present invention provides a pressure control apparatus for controlling the pressure at an output port without reducing the effective sectional area by controlling a main valve through the switching operation of a small-sized directional control valve, wherein the main valve has any of the following functions, i.e. a function of-providing a valve full-open position, a function of reducing pressure, and a function of providing a valve full-closed position. When a diaphragm chamber is placed at the secondary pressure by switching the directional control valve, the primary pressure is reduced by the main valve, and a predetermined reduced pressure is outputted. When the diaphragm chamber is at the primary pressure, the main valve is fully closed. When the diaphragm chamber is at the atmospheric pressure, the main valve fully opens, and hence the primary-side fluid flows to the output port as it is.