PURPOSE: To obtain a stable output signal with less distortion by identifying and regenerating a differentiation signal and making a pulse width of a pulse signal constant with a monostable multivibrator circuit.

CONSTITUTION: A signal 101 resulting from differentiating an input signal 100 by a differentiation circuit 10 is identified and regenerated respectively at two identification levels V+, V- to obtain two pulse signals 102, 103. Then monostable multivibrators (M.M) 20, 21 are operated respectively at the rise of the inputted pulse signals 102, 103 and output pulse signals 201, 202 whose width is constant. The signal 201 is inputted to a sawtooth wave signal generating circuit 12, from which a sawtooth wave signal 203 whose period is equal to a sampling pulse signal is outputted. A sample-hole circuit 13 receives the signals 203, 202, sample-holds the amplitude of the signal 202 to convert pulse width information into a voltage level signal 204. Through the such constitution, since the circuits 12, 13 receive a pulse whose pulse width is constant via the M.M 20, 21 a stable demodulation signal without distortion is obtained independently of the stability and accuracy of the circuits 10, 11.