PURPOSE: To adjust optionally the frequency change range of a frequency modulation outut by providing a level adjusting circuit adjusting an output level of an oscillator and allowing the output adjusted for the level to drive a voltage controlled oscillator.

CONSTITUTION: The level adjusting circuit 3 is provided with a transistor (TR) 30 to the base of which an output of the oscillator 1 is applied, a potentiometer 31 dividing a power supply voltage and an output terminal 35 or the like and an output generated from said output terminal 35 drives the voltage controlled oscillator 2. The TR30 is conducted or nonconductive by an output of the oscillator 1 applied to the base of the TR30 and a peak value of the output appearing at the output terminal 35 in this case depends on a voltage produced at a slider of the potentiometer 31 and the dip value depends on a variable resistor 32, then the output level produced at the output terminal 35 is adjusted optionally by adjusting the potentiometer 31 and the variable resistor 32.