A peak value detector detects power of an output light pulse which is output from the light amplifying fiber. A light receiving element receives a group of light pulses including a plurality of pulses and converts the group of light pulses into a current signal. The current/voltage converter circuit converts the current output from the light receiving element to voltage. The integration circuit integrates the voltage output from the current/voltage converter circuit. A programmable gain amplifier (PGA) amplifies the signal output from the integration circuit and provides the signal for the A/D converter circuit. The gain of the PGA is set by a gain setting signal from the signal processing circuit. The signal processing circuit adjusts the gain of the PGA so that the gain increases as the repetition frequency of the group of pulses increases.