A photo detecting device (5) has a photoelectric converting element (7) formed on a semiconductor chip with the photoelectric converting element (7) being divided into first and second light receiving regions (7a, 7b), and the photo detecting device further has a signal processing circuit formed on the same semiconductor chip, with the signal processing circuit including subtracting means (A1) for subtracting an output signal from the second light receiving region (7b) from an output signal from the first light receiving region (7a).
