PURPOSE: To prevent an electron from losing energy by colliding with an inner wall by arranging metallic bias plates electrically insulated from a chamber inner wall so as to cover the inner wall except the cusp tip, and imposing a negative bias on them.

CONSTITUTION: Lines 3 of magnetic force are generated by permanent magnets 2 on the outer periphery of a chamber 1, and a cusp magnetic field is formed. Circular arc-shaped bias plates 20 arranged between an inner wall of the chamber 1 and the cusp magnetic field so as to copy after a cusp shape are composed of a thin heat resistant metallic plate, and are supported on the inner wall with insulators. A negative bias is imposed on to the plates 20 by bias electric power supply 21, and they are put in negative electric potential to a wall surface. Plasma generated when a raw material gas is excited by a microwave is shut up in a central part by magnetic fields of the magnets 2 and electric fields of the plates 20. In this case, though electrons captured by the lines 3 of magnetic force are moved to the center along a spiral 23, a partial electron 26 passing through a magnetic field 27 is repulsed by the rear plates 20. Therefore, the electron shutting-up time becomes long, and collision with the wall is reduced.