PURPOSE: To obtain a checking method of a dehumidifier in a gas analyzing apparatus whereby the dehumidifying efficiency of the dehumidifier can be checked easily with high accuracy.

CONSTITUTION: A gas analyzing apparatus is provided with a sample gas line 4 with a dehumidifier 9 and a comparison gas line 5. A dry gas DG is supplied to the sample gas line 4 from the upper side of the stream of the dehumidifier 9. At the same time, the dry gas is fed to the comparison gas line 5 as well. The efficiency of the dehumidifier is checked based on the output of an analyzing part 2.