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) formed with an electric circuit, a light emitter and a light receptor mounted on the FPC and connected to an electric cable through the electric circuit, and a fixing tape 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 are provided around the light receptor 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.