Components of milk products are measured using near-infrared rays of two or more specific wavelengths from 700 to 1200 nm and a specific optical path length to 1 to 15mm. The two specific wavelengths may be one having a high correlation to the composition of an object component and the other having a close correlation, or preferably both having high correlations. An object is irradiated with light at the two specific wavelengths, and the amounts of light passing through it is measured to find light absorbancy, so that the composition of the object is evaluated using a double regression formula. The measurement is carried out using a detector which receives near-infrared rays from the Michelson interferometer optical system through a spectral filter and a sample in the sample cell, so as to convert its input into the electric quantity thereof to its input into calculate the light absorbancy of the object; a device that amplifies the light absorbancy; a device that digitally converts the amplified light absorbancy; and a device that operates the digitally converted electric signals.
