PURPOSE:To avoid the occurrence of moisture measuring errors due to the adsorption and desorption of moisture in flow paths, by separately providing a introducing ports of a sample gas and a zero gas to a crystal oscillating type sensor, introducing the gases through different flow paths, respectively, thereby switching the gases quickly. CONSTITUTION:A sample gas introducing path 20 is branched into two parts. One is guided to a zero gas flow path 25 of a crystal oscillation type moisture meter 24 through a closing valve 21, a moisture removing device 22, and a three way valve 23. The other is guided to a sample gas introducing port of the moisture meter 24 through a valve 26. A crystal oscillation type sensor 13 is provided in a cell of the moisture meter 24. The sample gas introducing port 11 and an introducing port 12 for zero gas, from which moisture is removed, are provided in the cell 10. When the zero gas is measured, the zero gas is introduced to the moisture meter 24 through ports (a) and (b) of the valve 23 and exhausted through the port 12. When the sample gas is measured, the port (a) is closed and the ports (b) and (c) are opened, and the sample gas is introduced through the port 12 and flowed to flow paths 30 and 29 through the port 11. In this way, substitution of both gases in the cell 10 is quickly performed, the flow paths are separated, and the occurrence of errors due to the adsorption and desorption of the moisture by the pipe wall is avoided. Thus the moisture in the sample gas is measured quickly and accurately.