A bio-photosensor used to examine the function of various organs of a human body or the like. It has a flexible printed circuit board (FPC) (1) formed with an electric circuit, a light emitter (2) and a light receptor (3) mounted on the FPC (1) and connected to an electric cable (15) through the electric circuit, and a fixing tape (5) adapted to be put on the FPC applied to a finger or the like to fix it in position. In order to support the emitter and the receptor, a strip of flexible film is used. It can be fitted on the finger comfortably. Pairs of sensors (A1-A4) are provided around the light receptor (3) to check if the receptor is put in an exactly opposite position to the emitter when the bio-photosensor is put on the finger.; The FPC is provided with a mark at such a position that the emitter and the receptor will be opposed to each other when the bio-photosensor is applied to the finger with the mark in contact with the finger tip.