PURPOSE: To improve FM modulation and to shorten set time by temporarily decreasing the time constant of a low-pass filter connected to a voltage control oscillator when the electric power source is put to work or when the frequency- dividing ratio of a variable frequency divider is varied.

CONSTITUTION: The FM modulating circuit is provided with voltage control type oscillator 1 composed of FET having the gate connecting with varactor diode 10 and an output signal appearing one output terminal 2a of it is supplied via prescaler 3 to programmable divider 4 whose dividing ratio can be changed by setting switch 11. Next, its output is applied to phase comparator 7, to which differentiating circuit 18, reference oscillator 5, fixed frequency divider 6, etc., are connected, to make a comparison with the output of divider 4 and its output is applied to low- pass filter 8 connecting with circuit 18, thereby applying amplifier 1 with its output as a control voltage with the time constant varied. Then, an FM-modulated signal appearing at the other output terminal 2b of amplifier 1 is applied to amplifier 14, whose output is sent out via output terminal 15.