When sulfur oxides are present in mercury vapor-containing gas, the adsorption of mercury vapor by activated carbon is inhibited. Therefore, there has been demand for development of a method for effective adsorption removal of mercury vapor even in the coexistence of sulfur oxides.Efficient and long-term removal of mercury vapor was made successful by contacting an activated carbon adsorbent consisting of 100 parts by weight of activated carbon impregnated with 5 to 70 parts by weight of only an alkali metal halide, with mercury vapor in sulfur oxide-containing gas.