To enable the amplitude modulation of an output waveform but while sensitizing to the amplitude of an optical signal waveform without sensitizing to the characteristic of the optical carrier signal of an optical signal by modulating an input signal according to the input of tap signals and forming an output waveform modulated dependently upon the coupling of the amplitude of the tap signal waveforms by this output signal.

The plural tap signals 20, 21 are formed by the optical signal and the individual tap signal waveforms B, C indicating the optical signal waveforms subjected to different individual delay periods by the respective tap signals 20, 21 are formed by the respective tap signals 20, 21. The tap signals 20, 21 are inputted as control signals to a modulating means (interference meter) 5. The optical input signal 50 is inputted to a modulation means 5, by which this signal is modulated. The input signal 50 is modulated by controlling the modulation means 5 according to the input of the tap signals 20, 21 and the optical output signal 22 is supplied. This optical output signal 22 forms the output waveform modulated dependently upon the coupling of the amplitudes of the tap signal waveforms B, C. The output signal 22 generally changes a cosine function with respect to the change of the optical signal 16.