3 at the outlet of gas cleaning tower 2 was 40 u00b0 c and the flow rate of the fluid circulated through gas cleaning tower 2 was 100 1 h the mercury containing absorbing fluid discharged from the water washing step was sprayed in a flash drum 20 the separated gaseous components were introduced into a mercury remover 4 where mercury was removed from the gas the amount of the gas flashed from waste water was 30 1n h on the other hand the gas freed of ammonia in the water washing step was fed to a hydrogen sulfide absorption tower 5 the gas temperature t 4 at the outlet of h 2 s absorption tower 5 was 40 u00b0 c and the flow rate of the fluid circulated through h 2 s absorption tower 5 was 30 1 h with respect to the above described system of fig2 hg concentrations were measured at various positions s 1 to s 6 in the system the results thus obtained are shown in table 1 below the difference between concentrations s 1 and s 2 given in table 1 i e s 1 s 2 is the amount of mercury which was removed by the water washing step moreover when the gaseous components separated by flashing waste water were passed through mercury remover 4 the mercury concentration was reduced from 3.