PURPOSE: To rapidly and accurately determine the concn. of a substance in gas by performing measurement under first and second gas pressures and calculating the concn. of a substance to be detected from the relation between the permeation intensity and the vol. concn. of a substance.

CONSTITUTION: At a time of the start of measurement, a valve 7 is opened to allow the introducing port 4 of a trough-shaped container 3 to communicate with a supply source of gas to be analyzed. When the pressure in the trough shaped container 3 reaches P1 or reaches the equilibrium state with the pressure of a gas supply source, the valve 7 is closed. A control device 8 records the intensity value of infrared rays detected by a detector 2 on an electronic measuring apparatus 9. Herein, the pressure detected by a sensor 10 and the temp. detected by a sensor 11 are recorded at the same time. Subsequently, a pump 6 is started and gas is discharged from the container 3 until pressure reaches P2. The pressure value P2 is almost half the pressure P1. When constant pressure P2 is obtained, the intensity value of detected infrared rays is recorded on the electronic measuring device 9 by the control unit 8. The concn. of the substance to be detected is confirmed on the basis of the theoretical relation between the permeation intensity and the vol. concn. of a substance by using a computer or a gas graph.