The present invention suppresses an adverse effect caused when an additional gas such as water vapor is mixed in a sample gas or the like that is subjected to gas measurement. In an embodiment of the present invention, in gas measurement for analyzing sensor output signals obtained by alternately supplying a sample gas and a reference gas to a sensor element while alternately switching between the sample gas and the reference gas, the sample gas and the reference gas pass through a humidity equilibration device partitioned by a water vapor permeable membrane, and then are supplied to the sensor element. As a result, since both gases have substantially the same value of humidity at the time of being supplied to the sensor element, influences of water vapor are substantially cancelled out in signals output from the sensor element by the alternate supply of the sample gas and the reference gas.