A processing gas constituted of C5F8, O2 and Ar achieving a flow rate ratio of 1≦C5F8 flow rate/O2 flow rate≦1.625 is supplied into a processing chamber 102 of an etching apparatus 100 and the atmosphere pressure is set within a range of 45 mTorr˜50 mTorr. High-frequency power is applied to a lower electrode 110 sustained within a temperature range of 20° C.˜40° C. on which a wafer W is mounted to raise the processing gas to plasma, and using the plasma, a contact hole 210 is formed at an SiO2 film 208 on an SiNx film 206 formed at the wafer W. The use of C5F8 and O2 makes it possible to form a contact hole 210 achieving near-perfect verticality at the SiO2 film 208 and also improves the selection ratio of the SiO2 film 208 relative to the SiNx film 206. C5F8, which becomes decomposed over a short period of time when released into the atmosphere, does not induce the greenhouse effect.