PURPOSE: To enable quantitative measurement even when an object to be measured has a foreign matter possibly absorbing or scattering light beside a substance to be determined, by measuring changes in the absorbance of light with several wavelengths before and after the state of the object to be inspected changes.

CONSTITUTION: Light with the intensity I0 is made incident on an object to be inspected such as living thing and the intensity I of the transmission light is measured to calculate absorbance A=logI0/I. Here, when a layer containing no desired substance in the object being inspected is represented by S, the desired substance in a tissue H containing the substance being represented by L and a remaining substrate left after the extraction of the desired substance L from the tissue H being represented by B, only the amount of the desired substance L in the object being inspected is varied to measure the absorbance again and thus, a variation ΔA in the absorbance before the change is determined. A plurality of changes ΔA are determined using lights with varying wavelengths to calculate the rate of change in the desired substance thereby enabling the quantitative determining of the desired substance L.