PURPOSE: To regenerate a signal with an accurate duty ratio at all times even when an amplitude of an input signal is changed by generating a threshold level used for the comparison with the input signal from the said input signal.

CONSTITUTION: An input signal enters an A/D converter 11, in which the signal is sampled at a prescribed period and converted into an n-bit digital data. The digital data as the n-bit digital data is fed to a maximum value detection section 12A and a minimum value detection section 12B. The maximum value detection section 12A and the minimum value detection section 12B detect respectively a maximum value and a minimum value of the input signal for one period and hold the detected values. The outputs from the maximum value detection section 12A and the minimum value detection section 12B are fed to an adder circuit 12C, its output is fed to a D/A converter 13, in which the output signal is converted into an analog signal. Then an output of the D/A converter 13 is used as a threshold level Vs.