In a gas chromatograph 1, a back-pressure calculation processing unit 242 calculates the back pressure of a first detector. A pressure calculation processing unit 243 calculates the pressure of carrier gas at a branch part. A backflow determination processing unit 244 compares the pressure of carrier gas at the branch part calculated by the pressure calculation processing unit 243 with the back pressure of the first detector calculated by the back-pressure calculation processing unit 242, and determines, when the pressure of carrier gas at the branch part is lower than the back pressure of the first detector, that the carrier gas flows back. Therefore, when there is a possibility that backflow of carrier gas occurs, the possibility can be assuredly recognized by checking of the result of determination by the backflow determination processing unit 244.
