PURPOSE: To measure moisture without any influence of variation in optical path length by using the absorption band of a material in paper other than the water for reference light.

CONSTITUTION: The absorption band of the material in the paper 2 other than the moisture is used for the reference light R and light with 1.94 μm wavelength is used as measurement light M; and the paper is irradiated with those light R and light M and the output ratio R/M is used as an output. The reference light R has the absorption band of cellulose, so when the paper is equal in the amount of cellulose and the percentagel of content of de-inked pulp(DIP) varies, the quantity of absorption varies correspondingly with the measurement light M. Therefore, even if the percentage of content of the DIP varies, the ratio R/M is held constant and does not vary. Consequently, the moisture meter which does not vary in output even by the variation of the DIP can be realized. Therefore, the moisture can be measured without any influence of variation in optical path length.