A contact hole, after hole etching, is subjected to light etching using a process gas containing a fluorocarbon-based gas and oxygen, with the oxygen being enriched, under condition without applying bias. Then, reaction products (5) having C—F bond and adhered to an interior of a hole (3) are removed using plasma treatment. After that, deposits (4) that have been left at a hole bottom are removed by wet processing. Then, a conductive material is buried in the hole to form a contact plug (7).