PURPOSE: To automatically diagnose clogging of a column, i.e., to prevent influences of the detection on the measuring time in the gas chromatograph when the pressure of a reducing valve is to be regulated with the use of a solenoid valve.

CONSTITUTION: In regulating the pressure of a reducing valve 6, an upstream solenoid valve 21 is driven and opened beforehand by a first driving signal VL and a secondary pressure P0 of the reducing valve 6 detected by a pressure sensor 7 is measured. The detecting value PV is compared with a reference value set for every column 3. Clogging of the column 3 is measured and diagnosed based on the comparing result. If the column 3 is clogged, the time constant of the rise of the secondary pressure of the reducing valve 6 is delayed. Therefore, the degree of deterioration of the column 3 can be detected by measuring the time constant.