In the forming process of buried wirings by filling wiring trenches formed in an insulator with a conductive film mainly made of Cu, the buried wirings are formed to have a uniform-height regardless of the width and density of the wiring trenches. When polishing a barrier conductor film comprised of a Ta film in the CMP process for forming the buried wirings, the polishing agent, which controls the removal rate of the underlying insulator of a silicon oxide film relative to the barrier conductor film to almost one twentieth or less, is used as the slurry, and the pad which is made of polyurethane with a hardness of 75 degrees or more measured by the Type E durometer in conformity with the JIS K6253 and which is comprised of the foam including non-uniform pores with a diameter of about 150 .mu.m or larger and a density of about 0.4 0.16 g/cm.sup.3, is used as the polishing pad.