PURPOSE: To obtain a pulse without being influenced by the Am fluctuation of an input signal including an AC component by providing 1st and 2nd comparators and a pulse generator to a waveform shaping device generating a pulse from the input signal including the AC signal.

CONSTITUTION: A gate circuit inhibits the transmission of an output signal (b) of the 1st comparator 1 to a flip-flop circuit when the output signal (c) of the 2nd comparator 2 is at a high level. Thus, an output signal (d) of the pulse generator 3 is set to a high level when the output signal (c) of the 2nd comparator 2 is at a high level and reset to a low level when the output signal (b) of the 1st comparator 1 goes to a high level after the output signal (c) of the 2nd comparator. 2 goes to a low level. Thus, the output signal (d) is a pulse signal having a trailing edge at the leading of the AC 0 level of the input signal (a). As a result, the period of the trailing of the output signal (d) is not influenced by the AM fluctuation of the input signal (a).