A light detecting device is fabricated to have the following structure: a light receiving element having a protective layer, in which a transparent conductive electrode, a semiconductor layer, an electrode and the protective layer are successively formed on a transparent substrate, is arranged on a first face of an insulating substrate having a terminal electrode formed to be exposed to first and second faces of the insulating substrate in such a manner that the protective layer is opposed to the first face of the insulating substrate; and further the transparent conductive electrode and the electrode are connected electrically to the terminal electrode exposed to the first face of the insulating substrate. The light detecting device having this structure is surface-mounted on a circuit board to connect the terminal electrode exposed to the second face of the insulating substrate to an external terminal of the circuit board.