PURPOSE: To desulfurize a waste gas with magnesium hydroxide as a absorbent by roughly removing SO2 in a preceding stage and highly desulfurizing in a poststage so as to be equal to below prescribed content in the outlet SO2 in two stage absorption columns.

CONSTITUTION: Absorption columns 3, 4 are installed in two stages and a separator 7 for preventing the mixing of a liquid absorbent slurry circulating in each of the absorption columns 3 and 4 is formed at a slurry trap part 6 in the bottom of the absorption column. And an absorbent consisting of a low pH magnesium based slurry and an absorbent consisting of a high pH magnesium base slurry are used in the preceding stage absorption column 3 and in the succeeding stage absorption column 4 respectively. The major part of the sulfur dioxide gas in the high concn. sulfur dioxide containing gas is absorbed and removed in the preceding stage absorption column 3 and the residual sulfur dioxide gas in the sulfur dioxide containing gas made low concn. is absorbed and removed in the succeeding stage absorption column 4.