the present invention is capable of separating / removing the influence of skin blood flow contained in near infrared spectroscopy 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 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 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 .