A D-class amplifier includes: a bridge circuit adapted to drive an inductive load; a power supply voltage detection section outputting a quantized power supply voltage signal indicating a power supply voltage fed to the bridge circuit; and a gain-controlled PWM section adjusting a gain in response to the quantized power supply voltage signal, amplifying the input signal in response to the gain, generating a PWM signal from the amplified input signal, and feeding the PWM signal to the bridge circuit. The power supply voltage detection section includes: an error integration section generating a quantized signal by integrating a difference between the power supply voltage and the quantized power supply voltage signal; and a digital filter removing high frequency components of the quantized signal to output the quantized power supply voltage signal. The gain-controlled PWM section controls the gain such that variations of the power supply voltage are cancelled.