Abstract: A phase adjusting circuit for adjusting phases of output powers from two high frequency power generators in a plasma processing apparatus comprises first and second synthesizing circuits for generating first and second high frequency signals of a predetermined waveform with frequencies of f0 and f0.+-..DELTA.f (.DELTA.f<<f0), a phase difference detection circuit for detecting a phase difference between detection signals of plate electrodes, a third waveform synthesizing circuit for generating a high frequency signal with frequency and waveform same as the first frequency signal which has a phase determined from a phase error between a set phase difference and an output from the phase difference detection circuit and a phase adjusting circuit which determines output powers of the two high frequency power generators base on outputs of the first and third waveform synthesizing circuits.