A semiconductor device includes a photoelectric conversion element outputting an electric signal in accordance with an externally input optical signal, an internal circuit processing the electric signal, a signal terminal inputting or outputting a signal to the internal circuit, a first voltage terminal supplying a first voltage to the internal circuit, and a second voltage terminal supplying a second voltage lower than the first voltage to the internal circuit. The semiconductor device further includes a first protection element connected in an electrically reverse direction between the signal terminal and the first voltage and having at least one PN junction and a second protection element connected in an electrically reverse direction between the first voltage and the second voltage and having at least one PN junction, so that immunity to electrostatic noise can be improved and electrostatic breakdown and malfunction of the photoelectric conversion element can be suppressed.