PURPOSE: To achieve the cost reduction of apparatus of a gas chromatograph by enabling the measurement and control of the flow rates of a plurality of carrier gas passages to be performed and making the number of carrier gas flowmeters installed one.

CONSTITUTION: When the flow rate of the carrier gas in either one of carrier gas passages 10, 20 is controlled by either one of carrier gas flow rate controllers 11, 12, pressure fluctuations are generated in the controlled passage and the pressure value of carrier gas is inputted to a passage judging device 3 from a pressure sensor 12 or 22 and it is judged that which of the flow passages pressure fluctuations are generated in, that is, in which of the passages a flow rate is controlled, by a passage judging device 3 and the controlled carrier gas passage is automatically selected by a passage changeover device 1. The flow rate of the carrier gas of the selected passage is controlled while being measured by a carrier gas flowmeter 2.