PURPOSE: To adjust the density of a plasma generated and the ratio of its compo sition in accordance with processing conditions, by impressing a high-frequency voltage through the medium of an amplifying means for amplifying a composite high-frequency voltage to one of upper and lower electrodes, and grounding the

CONSTITUTION: A semiconductor wafer W being a substance to be processed is put on a susceptor 7 in a processing container 2, and attracted and held with an electrostatic chuck 11. Following this, a plasma is generated by impressing a specified composite high-frequency voltage generated by two high-frequency power sources 21 and 22 composed with a mixer 23, and amplified with an amplifier 24 between the upper electrode 18 and a susceptor 7 constituting the lower electrode, and along with this, a process gas is flowed from the side of the upper electrode 18 into a processing space to perform etching processing. On that occasion, optimum etching processing becomes feasible, by selecting high-frequency to be composed the most appropriately, an adjusting the ratio of composition of the plasma the most appropriately.