Abstract: A photosensor provided with a transistor having a bias element connected between the emitter and the collector, a light emitting element connected in series to said transistor, and a photosensitive element for receiving light emitted by said light emitting element and transforming it into electric current, with one end connected with said transistor. Owing to this construction, since only two interface lines are needed, one being the line for feeding the transistor with electric power and the other being the line of taking out the output of said photo sensitive element, the number of interface lines is reduced and thus the realization is simplified. Since the current flowing through the photosensitive element is so controlled that it is amplfiied and applied to the light emitting element, the output of the photosensitive element is considerably increased.