PURPOSE: To receive the input signals with no error by securing the follow-up to the first significant input signal.

CONSTITUTION: A switch part 8 is provided between a peak detector 5 and a comparator 6 and at the same time the part 8 is switched by a signal presence detecting part 9 set at the input side of a post-amplifier. When no optical signal Pin is supplied, the part 8 is switched to the side of the initialized voltage VSET by the initialization system switching signal EXa sent from the part 9. Then the gain between the redoubling factor and the amplification factor is set previously at the optimum value as shown by an arrow head (a). When the signal Pin is supplied, the part 8 is switched to the side of the detector 5 by the automatic gain control system switch signal EXb sent from the part 9. Thus the feedback can be applied as shown as arrow mark (b) of a broken line. In such a way, the conventional transient answer time is omitted since both the redoubling factor and the amplification factor are already set at each optimum value. Thus it is possible to discriminate accurately the first and subsequent input signals.