A class-D amplifier according to an embodiment includes a PWM modulator, a first output transistor group that includes two transistors complementarily operating and includes a first connection point between the two transistors as an output terminal, a second output transistor group that includes two transistors complementarily operating and includes a second connection point between the two output transistors as an output terminal, and a selector configured to selectively provide a PWM pulse signal to one of the first output transistor group and the second output transistor group. A system that includes the second output transistor group, a low-pass filter, and a load connected to the low-pass filter configures a series resonance circuit.