A method of production of activated carbon for removal of mercury gas which provides activated carbon impregnated with both sulfur and iodine which gives a higher mercury gas adsorption performance compared with a conventional activated carbon adsorbent and also enables the prime cost of manufacture to be kept down, that is, a method of production provided with a sulfur impregnation step which adds sulfur to activated carbon and heats the mixture to obtain sulfur-impregnated activated carbon comprised of activated carbon to 100 parts by weight of which sulfur is impregnated in 5 to 20 parts by weight and, after the sulfur impregnation step, an iodine substance impregnation step which adds an aqueous solution containing iodine and an iodine salt to the sulfur-impregnated activated carbon.