the high frequency electric power was applied to the cathode electrode 4 at the frequency of 27 12 mhz and the power density of 0 67w cm 2 when the film forming chamber 10 was cleaned by the radicals and ions the high frequency electric power was applied to the cathode electrode 4 at the same frequency as that of the cleaning only by the ions and at the half power density of that of the cleaning only by the ions the cleaning rate of the cleaning operation only by the radicals are low however that of the cleaning operation by combination of the radicals and ions is substantially equal to that of the cleaning operation only by the ions the required power of high frequency in the cleaning operation by combination of the radicals and ions is about half of that in the cleaning operation only by the ions accordingly the plasma damage to the shower plate 5 can be reduced and so the shower plate 5 can be prevented from being deteriorated next there will be described a third embodiment of this invention parts in this embodiment which correspond to those in the first and the second embodiments are denoted by the same reference numerals the detailed description of which will be omitted 0043 fig3 shows a vertical cross sectional view of a plasma film forming apparatus 30 according to this embodiment fig4 shows a cross sectional view taken along the lines iv iv in fig3 it is used for a large sized substrate in the first and the second embodiments as shown in fig2 the radicals are introduced laterally into the film forming chamber 10 accordingly portions nearer to the outlet of the gas introducing pipe 23 are sooner cleaned when the size of the substrate 9.