PURPOSE: To minimize diffusion of components of a sample, by backflushing the first column by use of a carrier gas effused from the second column in a backflushing time.

CONSTITUTION: In a gas chromotograph, a sample to be determined is transferred by a carrier gas to the first column 3 via one and 3a thereof, and is separated into its respective components in a flow line shown in (a). Then some components are eluted, and transferred to the second column 4 via one and 4a thereof. Before the remaining components flow into the column 4, the flow is changed over to another flow line shown in (b). Thus the first stage, i.e., the stage of partial separation of components, is finished, and the second stage, i.e., the stage of backflushing, is started. The flow direction of the carrier gas is reversed, and the remaining components in the column 3 are backflushed and eluted from the column 3 as a whole, and then transferred to a detector 5. The concentration of the remaining components is detected as a whole, and recorded as a signal peak in a recording part 6.