A method for measuring water content in a compound using a 3-wavelength infrared water content tester uses a reference wavelength combination in which a reference wavelength [Lambada]L on the long wavelength side of a wavelength [Lambada]W that is absorbed by water and a reference wavelength [Lambada]S on the short wavelength side of the wavelength [Lambada]W are a reference wavelength pair, and b in the relational expression k = aw + b between the water content w in the compound and the light absorbance k thereof is viewed as zero within the range of the allowable measurement error of the infrared water content tester, regardless of the compounding ratio in the compound.