PURPOSE: To enable concn. measurement with high accuracy in low concn. under high noise, by automatically performing such control that chopping frequency is received in a resonance frequency band region.

CONSTITUTION: Lead filters 18, 19 have characteristics wherein an inherent resonance frequency band region is extremely narrow and the resonance frequency band region selects noise out of frequency band regions of various noises. In addition, the chopping frequencies of choppers 15, 16 are controlled so as to be made same to the resonance frequencies inherent to oscillation lead filters 30, 31 and signal waves having frequencies same to that of the selected filters 18, 19 are oscillated from an object to be measured. As a result, even if noise is high and gas to be measured low in the concn. of an object to be measured is a target, the concn. measurement of the object to be measured can be performed with high accuracy by utilizing simple amplifiers 22, 23.