In a transmission power amplifier apparatus, a signal calculator divides an input signal into a plurality of N constant amplitude signals having a substantially identical predetermined constant amplitude value and having predetermined phases different from each other, and N power amplifiers amplify electric powers of the N constant amplitude signals under same predetermined operating condition. A power combiner combines the electric powers of the N power-amplified constant amplitude signals, and outputs a combined output signal. A controller controls the signal calculator to detect an amplitude value of the input signal, to detect an average value and one of a maximum value of the amplitude value of the input signal and a peak-to-peak value of the amplitude value for a unit time interval based on the detected amplitude value of the input signal, and to divide the input signal into the N constant amplitude signals having a constant amplitude value and different phases.