PURPOSE: To enable surface treatment little in dispersion at every base plate by leading plasma generated by microwave onto the base plate and controlling RF output for impressing bias onto the basis of capacitance between the base plate and the bias electrode.

CONSTITUTION: Gas is introduced into a plasma chamber 13a of a vacuum vessel 13 and plasmaized by resonance effect of both microwave introduced from a waveguide 1 and the lines of magnetic force of an exciting solenoid 4 arranged to the circumference. This plasma is guided along the above- mentioned lines of magnetic force and introduced into the reaction chamber 13b of the vacuum vessel 13. On the other hand, bias is impressed by an RF generating means 17 via an RF electrode 21 which is oppositely arranged via a base plate 15 arranged in the reaction chamber 13b and an insulator 22. Thereby, active atoms, molecules and ions in the plasma are accelerated and the base plate 15 is etched or a thin film is formed. In the above-mentioned plasma treating device by microwave, strength of capacitance connection of both the base plate 15 and the RF electrode 21 is measured by a capacity sensor 19. The output of the RF generating means 17 is corrected by the output signal thereof. Thereby dispersion of surface treatment at every base plate is reduced.