A signal processing device for processing an information signal, comprises a frequency modulator arranged to receive the information signal, to frequency-modulate the received information signal and to output a frequency-modulated information signal, a feedback signal generator arranged to receive the frequency-modulated information signal output from the frequency modulator means and to generate a feedback signal therefrom at a phase which is variable to be either in-phase or out-of-phase relative to the received frequency-modulated information signal in accordance with a duty in a high-level period of the received frequency-modulated information signal and at an amplitude which is variable within a range from a zero level to the same level as the received frequency-modulated information signal, and addition circuitry arranged to add the feedback signal output from the feedback signal generator to the information signal received by the frequency modulator.