PURPOSE: To raise the frequency of a carrier signal with a simple constitution, by making the time constant of a time constant circuit variable to change the degree of dullness of the waveform in accordance with a modulation input, and comparing the waveform with a fixed threshold.

CONSTITUTION: When a time constant coefficient K of a time constant circuit 2 consisting of a resistance element 21 of a resistance value R and a variable capacity element 22 of a capacity value C is denoted as K, a time constant T is expressed by equation T=1/KCR. Consequently, the capacity value C is made variable to make the time constant variable, and the capacity value C is controlled by a modulation input signal 4 to change positions of the rise and the fall of a carrier input signal 1 in accordance with the signal 4. This dulled waveform passes through a fixed threshold circut 31 to obtain a modulation output signal 5 having a pulse width and pulse positions corresponding to the signal 4.