PURPOSE: An apparatus is provided to enable rapid sedimentation of an oxidized mercury into a gas/liquid interface in a scrubber by adding hydrogen sulfide and/or an aqueous sulfide species to an industrial gas in the scrubber, when the industrial gas containing the oxidized mercury is received and cleaned using a scrubber solution. CONSTITUTION: A boiler installation(10) has a furnace(12) with a burner(25) attached to which fine powder coal(24) is supplied from a finely crushing machine(22), and a flue gas generated in this furnace(12) is sent to an air heater(18) through a gas outlet(14). Next , the flue gas is sent to an electric dust collector(26) and a wet scrubber absorbing device module(30). Further, the gas is cleaned to be discharged into a smokestack(32). A wet scrubber(30) pumps up a scrubber solution(38) stored at the bottom part using a circulating pump(40) and makes the solution(38) spew out from a spray header(44) to absorb SO2 from the flue gas(16). In this case an H2S generation system(50) is provided and H2S is injected from an injection system(76) at the front stage of the wet scrubber(30). Thus it is possible to facilitate the remaining mercury removing treatment.