PURPOSE: To exactly perform concentration measurement independent of fluctua tion of a base line.

CONSTITUTION: Temperature in an analyzer body is kept at a set point. Carrier pressure is kept at another set point. Operation sequence is read and analysis operation is started. Thereby a predeterminated gate time band is set every gas component to be measured for measured density signal and a retention time Rt of chrchromatogram of the gas component to be measured appearing in the gate time band. A density value in accordance with the retention time Rt of a measured gas component to be measured is found from an analytical curve indicating relation of the retention time Rt and the concentration predeterminated every gas component to be measured.