To provide a power amplifier and a power amplifying method which control the output of a plurality of amplifying systems which amplify an input signal and synthesize its output by a power synthesizer to obtain an output signal to the same amplitude and the same phase.

An average value of output signals of amplifying systems 13A and 13B is obtained by an average value detector 23 by digital calculation processing, the average value is compared with a fixed value 26 common to the amplifying systems 13A and 13B by a comparator 24, and the bias voltage of an amplifier 14 in the amplifying systems 13A and 13B is controlled based on the comparison result.