PURPOSE: To improve uniformity of plasma and enhance the efficiency of microwave utilization by installing a magnetic field generating means in the neighborhood of an electrode in the form of a flat plate, and supplying microwaves through a circular waveguide tube to a slit provided in the electrode.

CONSTITUTION: A mechanism 10 is to hold the inside in a decompressed state by an exhaust port, and a vacuum vessel 1 is equipped with this mechanism 10 and a mechanism to introduce a gas. In the vessel 1 a plasma generating mechanism and a base board holding mechanism 3 are installed opposingly at a certain spacing to the mechanism 2. Microwaves are introduced to the vessel 1 and jetted in it through a slit provided in an electrode 4 in the form of a flat plate. A magnetic circuit 6 as a magnetic field generating means and a circular waveguide tube 7 to supply the microwaves to the slit are provided in the neighborhood of the electrode 4. The electric field generated by the microwaves and the magnetic field produced by the circuit 6 make interaction, and the introduced gas is turned into plasma, and the surface of the base board 5 undergoes a processing. Thereby the uniformity of the plasma is improved, and the efficiency of the microwave utilization of the electrode for discharging is enhanced.