A front-end processing circuit applies predetermined processing to an input signal and outputs a processed signal. A frequency modulation circuit applies frequency modulation to the output signal of the front-end processing circuit and outputs a processed signal. The front-end processing circuit includes a variable amplifier and a level detector. The variable amplifier receives a feedback signal and amplifies a signal by an amplification factor corresponding to the feedback signal. The level detector outputs the feedback signal to the variable amplifier in accordance with a level of a signal obtainable in a circuit succeeding the variable amplifier. The front-end processing circuit maintains the signal supplied to the frequency modulation circuit at a constant level.