A digital amplitude modulation apparatus is provided with a measuring section, a fast surge protection section, a slow vector protection section, and a controller. The measuring section detects a voltage and a current from an RF signal. The fast surge protection section calculates an SWR from a detection signal of the measuring section, and obtains a first upper limit of the number of power amplifiers which can be put into an ON state based on the SWR and generates a first control signal. The slow vector protection section obtains a reflection coefficient from the output detection signal of the measuring section, and obtains a second upper limit of the number of the power amplifiers which can be put into an ON state based on the reflection coefficient and generates a second control signal. The controller controls the power amplifiers to be ON/OFF.