An accurate measurement can be made of the light absorbance of deep layer tissue such as in a human body or a fruit. The thickness of fat is computed. A first specific distance and a second specific distance corresponding to the computed fat thickness are computed based a predetermined relationship between the fat thickness, the first specific distance, and measurement sensitivity of a surface layer and measurement sensitivity of a deep layer when light is received at a position the first specific distance away from a light emitting means. A third specific distance and a fourth specific distance are computed corresponding to the computed fat thickness based on a predetermined relationship between the fat thickness, the third specific distance and a measurement sensitivity of an intervening layer and a measurement sensitivity of the deep layer when light is received at a position the third specific distance away from the light emitting means.