At a surface of a semiconductor wafer W as a processing object, a SiOC layer, a SiC layer and a Cu layer are formed in the order from an upper side. In the SiOC layer, a first opening for forming a via is formed. Using the SiOC layer as a mask, a plasma etching of the SiC layer is selectively carried out by using an etching gas containing a gas mixture of NF.sub.3/He/Ar, thereby a second opening continuously following from the first opening being formed. Therefore, it is possible to carry out the etching of the SiC layer with a high selectivity with respect to the SiOC layer.