Light (P) of a given wavelength is incident onto one light incident position (I) located on the surface (Ba) of a scattering-absorption body (B) and the light (P) propagating through the inside of the scattering-absorption body (B) is detected at one light detection position (D) located on the surface (Ba) of a scattering-absorption body (B) to produce a light detection signal. A time waveform in association with the light intensity of the detected light is obtained on the basis of the light detection signal, and the average length of the light path (L) of the light (P) within the inside of the scattering-absorption body (B) and the information associated with the amount of substance to be measured in a measuring region (B1) are calculated on the basis of the time waveform. On this occasion, the information associated with the amount of substance to be measured is corrected based on the average length of the light path (L) such that the longer the average length of the light path (L), the greater the amount of substance to be measured becomes. According to this method, measuring results free from the effects of interposing tissues can be obtained using a simple method.