According to one embodiment, a digital amplitude modulator includes a power supply unit, a measurement unit, a calculation unit, and a power controller. The power supply unit supplies a power supply voltage to a plurality of power amplifiers. The measurement unit measures the output power and reflection coefficient of a synthesizer. The calculation unit calculates loss power occurring in each power amplifier in an ON state, and reflected power consumed by each power amplifier in the ON state. The power controller controls all the power amplifiers to be in an OFF state, and performs a normal operation, and controls the input voltage of the power supply unit or ON/OFF of the plurality of power amplifiers.