PURPOSE: To measure the quantity of pitch in a raw material and a process by performing colorimetry for the pitch in the manufacturing process by using one or more specified wavelengths within a specified range.

CONSTITUTION: Light in the region of near infrared rays in the range of wavelengths 0.8 - 2.6 μ gives absorption spectrums peculiar to compounds. The light can be used for the identification and determination of a compound. The spectrums have overlapped absorption bands. Differentiated spectrums are used for processing the bands. Thus, gentle change due to the difference in surface properties, the difference in colors and the like can be erased from the complicated spectrums. The absorption peculiar to the component of pitch can be extracted. Each peak is proportional to the quantity of the component, and the quantity can be determined accurately. The absorption peculiar to the component of the pitch among the near infrared rays is indicated. The light having the wavelength whose quantity can be determined is selected. The correlation with respect to the absorbances of the spectrums and the peak intensities of the differentiated spectrums and the quantity of the pitch is obtained for one or more lights. This expression of the correlation is used, and the quantity of the pitch is determined based on the intensity of the peak.