PURPOSE: To enable a measurement and analysis at a high S/N ratio by specifying comparision gas.

CONSTITUTION: As comparison gas supplied between magnetic pieces 2 and 2', and 3 and 3' and filling left and right chambers CA and CB of capacitor microphone detector 7, hydrogen gas, helium gas or their mixed gas is used. Then, measured gas is supplied from intakes 4 and 4' between magnetic pieces 2 and 2', and 3 and 3', and comparison gas from intake 5 and 5'. Next, generated interface pressure is transferred to capacitor microphone detector 7 and when the oxygen concentration of the measured gas is read by meter 10, gas impact to capacitor film 7a is decreased by specifying the comparison gas to reduce noises, so that a measurement and analysis may be taken at a high S/N ratio.