A power amplifier according to the embodiments includes: a silicon substrate; an input terminal configured to receive an input of a RF signal; a power dividing unit configured to divide the RF signal into a first signal and a second signal; a phase modulating unit configured to modulate a phase of the second signal; an N well formed in the silicon substrate; a P well formed in the N well and configured to receive an input of the second signal of a modulated phase; a gate insulating film formed on the P well; a gate electrode formed on the gate insulating film and configured to receive an input of the first signal; source and drain electrodes formed on both sides of the gate electrode in the silicon substrate; and an output terminal configured to output a RF signal obtained from the drain electrode.