PURPOSE: To hold the change of an ASK modulation degree at a constant value by correcting the change of a response level accompanying the fluctuation of the quantizing efficiency of an exciting light source by a simple circuit constitution, and to make a circuit scale compact.

CONSTITUTION: This device is equipped with an exciting light source 9 which outputs an exciting light, light source driving circuit 10 which drives the exciting light source 9, oscillator 16 which generates a response frequency for ASK- modulating the exciting light, WDM coupler 5 which multiplexes the exciting light and an optical signal, rare earth dope fiber 3 which amplifies the optical signal by the exciting light, optical branching circuit 8 which branches the optical signal and a frequency for monitor transmitted from an optical terminal station device, direct current amplifier 12 which direct current-amplifies the optical signal, and outputs a control voltage, and and BPF 19 which extracts the frequency for monitor. The device is equipped with a superimposing circuit 15 which PWM-modulates a response frequency by in-repeater information (x) according to the frequency for monitor, and a gain variable multiplier 13 which amplitude-controls the PWM modulated signal by the control voltage outputted from the direct current amplifier 12.