The present invention is capable of separating/removing the influence of skin blood flow contained in near infrared spectroscopy (NIRS) signals and extracting a brain- or brain cortex-origin signal. Moreover, the present invention enables versatile separation of brain-origin and skin-origin signals in view of differences among individuals. A biological photometric device, wherein light transmitters and light receivers are located in such a manner that measurement can be conducted at a plurality of source-detector (SD) distances and light received by the individual light-receivers can pass through the gray matter to thereby separate a brain-origin signal and a skin-origin signal. Individual component analysis (ICA) is conducted on data obtained at the individual measurement points. Then, it is determined whether each individual component originates in the brain or in the skin with the use of the SD distance-dependency of the weighted value of each of the separated components.
