There is provided a power amplifier that can maintain a constant gain by detecting a level of a signal being input and a level of a signal being output. A power amplifier according to an aspect of the invention may include: an amplification section having at least one amplification unit amplifying an input signal according to an adjustable gain to thereby output the amplified input signal; a detection section detecting signal levels of an input signal and an output signal of the amplification section; and a gain maintaining section controlling a bias power according to a detection result of the detection section so that a gain of the amplification section is maintained within a predetermined gain range.