PURPOSE: To restrict the impurity ion of a cathode material from mixing into a discharge plasma by providing a sputter shield at an anode that is opposed to a cathode and at the reverse position.

CONSTITUTION: An impurity (cathode material) ion is prevented from entering a plasma space 7 by providing a sputter shielding plate 10 which prevents the impurity ion and condensing the vapor and ion evaporated from the cathode. The sputter shielding plate 10 is fixed by an insulating plate holding section 11. In the figure, the potential of the sputter shielding plate is the same as the cathode poential, but it is not always restricted to this ptential. For example, if the sputter shielding plate is set to the more negative potential than the cathode, an electron coming from the cathode is returned to the anode and said plate is used more effectively during discharge. Besides, since said plate attracts he impurity ion, the impurity ion can be condensed effectively.