PURPOSE: To decrease a distortion imparted to data, in the case a frequency modulated wave is generated directly by modulation data having a DC component, with regard to the frequency modulating circuit, for instance, used for a data transmitter.

CONSTITUTION: In the frequency modulating circuit having an oscillating/phase comparing means 1 and a band limiting/synthesizing means 2, together with generating directly a frequency modulated wave by using modulation data containing a DC component, a degree of balance of H level data and L level data in the modulation data concerned is detected. This circuit is constituted by providing a switch control means 3 for sending out a switch control signal at the time when it is decided that the detected degree of balance is outside of a range set in advance, and a switching means 4 for disconnecting a connecting line between the oscillating/phase comparing means and the band limiting/synthesizing means concerned.