PURPOSE: To simplify the gain changeover operation by providing a comparison circuit comparing the average light receiving signal composed of the dc component of a light receiving element with the reference value given in advance.

CONSTITUTION: When a large optical input comes in, the large average light receiving signal can be obtained from a light receiving signal. When this average light receiving signal exceeds the reference value given in advance, the output of a comparison circuit 5 actuates and changes a gain changeover means 4 and the gain is reduced. On the contrary, when the light input is small, the average light receiving signal is lower than the reference value so that the output of the circuit 5 actuates and changes the changeover means 4 and the gain is increased. Thus, since the gain is switched based on the light receiving signal, the gain is small by switching the gain before amplified or saturated when the light input changes from small to large input. Accordingly, the saturation of the amplifier circuit 3 can be prevented.