PURPOSE: To obtain a normally accurate moisture value even if an object to be measured varies in a sort, by a method wherein a ratio signal of a sum of two comparision signals to a measuring signal is found, and the ratio signal is corrected by the ratio between said two comparing signals.

CONSTITUTION: A ratio signal 2S/(R1+R2) of a first signal S of a transmitting light or reflecting light from an object to be measured in an absorbing wavelength zone of moisture to a sum signal R1+R2 of a second signal R1 and a third signal R2 of transmitting light or reflecting light from objects to be measured in comparing wavelength zones at both sides of an absorbing wavelength zone of moisture, is found, and the ratio signal is corrected by the second signal R1 and the third signal R2. Provided a correcting function is f(R1/R2), the ratio signal is [2S/(R1+R2)].f(R1/R2), and if a correcting function is previously found from experiment or the like, a correction is made by the result acting as an identification discriminating signal, and this permits moisture to be normally accrately measured not according to a sort.