Operation of a digital power amplifier for power amplification of a modulated signal is stopped in a period in which a voltage value of a drive signal applied to a capacitive load is constant, to thereby suppress power loss. The power amplification is stopped either when half a period of time when the modulated signal in a first voltage state maintains the first voltage state elapses or when half a period of time when the modulated signal in a second voltage state which is lower in voltage than the first voltage state maintains the second voltage state elapses. Accordingly, when electric current does not flow in a inductor of a low pass filter, it is possible to stop the power amplification. Thus, it is possible to prevent generation of voltage fluctuation in the drive signal due to an electromotive force caused by a self-induction phenomenon of the inductor.