A photosensor for realizing an image sensor which can meet the requirements of high resolution, high operation speed and high signal-to-noise ratio is disclosed. The photosensor comprises a circuit substrate, a thin film transistor formed on the circuit substrate and an amorphous silicon photodiode formed on the substrate integral with the thin transistor between the drain and gate electrodes thereof. Also formed on the circuit substrate adjacent to the thin film transistor and photodiode are a charging switch element for coupling the photodiode to a DC power source to charge an inter-electrode capacitance of the photodiode, a charge storage capacitor charged by a channel current of the thin film transistor controlled by an inter-electrode capacitance voltage of the photodiode which varies in response to incident light after the inter-electrode capacitance has been charged, and a detecting switch element for coupling the capacitor to an output amplifier. The charging and detecting switch elements are each formed of a thin film transistor.