PURPOSE: To prevent the generation of a noxious gas during the etching of a thin gold film vapor-deposited on a substrate and to facilitate the handling of an etching soln. and the treatment of waste liquor by using an aqueous soln. of an alkali metallic sulfide as the etching soln. for the thin gold film.

CONSTITUTION: A thin gold film vapor-deposited on a substrate is etched with an aqueous soln. contg. ≥0.05wt% alkali metallic sulfide such as Na2S, K2S or Li2S. By this method, the film can be etched easily. Unlike etching with aqua regia, any special vessel for the etching soln. is not required, no noxious gas is generated during the etching, and the waste liquor is treated easily. Said etching soln. gives a finish equal or superior to a finish given by aqua regia.