Carbon-sulfur-gold (C-S-Au) composite thin films were formed by a co-operation process of plasma CVD and sputtering, using methane, SF, and Ar mixture gas, and gold plate on the discharge electrode. The content of Au atoms in the film was smaller than 2 at.%. The refractive indexes were measured to be 1.6, 2.4 and 3.7 for C, C-S (80:20 at.% ratio) and C-S-Au (81.1:18.8:0.1 at.% ratio), respectively. The electronic polarizabilities of C, S and Au are discussed, using the values of refractive index.