A photo diode and a signal processing circuit are formed on a silicon substrate. The signal processing circuit comprises a PNP transistor and an NPN transistor. A region of the signal processing circuit on the silicon substrate is covered by an aluminum thin film functioning as a shielding film. A covered distance L(.mu.m) is defined as an overhang of the aluminum thin film from the edge of the PNP transistor, and is determined based on a ratio of a minimum current of the PNP transistor, which induces malfunction in the signal processing circuit under the solar radiation, to a current generated in the circuit element when subjected to the solar radiation without the aluminum thin film.