PURPOSE: To achieve automation of a calibrating operation in a gas chromatography by measuring a peak position of a reference gas beforeh and to change a set value of a pressure reducing valve automatically based on comparison results between the resulting measured value and a reference value.

CONSTITUTION: When a detection signal PV of a pressure sensor 7 and a carrier gas pressure setting signal SP from a CPU23 are inputted into a carrier gas pressure control section 25, the control section 25 opens or closes solenoid valves 21 and 22 based on the comparison results thereof to control a pressure PO on the secondary side constantly to a set value. In this case, relationship among a primary side pressure PS, a back pressure PN and a secondary side pressure PO is PS≥PN>PO. For example, when the detection signal PV of the sensor 7 is below a lower limit value of the pressure set value SP, an upstream side solenoid valve 21 is opened by a signal from the control section 25 while a downstream side solenoid valve 22 is closed to raise the back pressure PN of a back pressure section 11b. The back pressure PN pushes down a diaphragm 12 against a compression spring 16 to open a poppet valve 15 and a gas CG flows into a pressure chamber 11a to increase the secondary pressure PO.