PURPOSE: To measure SO2 density with high accuracy by an apparatus and a method easy in maintenance wherein sample gas deprived of NH3 through a partition wall which passes NH3 gas selectively therethrough, is absorbed into a solution which absorbs SO2, and then conductivity of the absorbing solution is measured.

CONSTITUTION: The air, exhaust gas, etc. containing SO2 is introduced to a filter 2 from a pipeline 1 to remove dusts, etc., and then is introduced to an NH3 removing means 20 including a cylindrical partition wall 5 which passes MH3 gas selectively therethrough and which is disposed concentrically within a cylindrical container 4. The partition wall 5 is formed of ion changing resin which has a sulfonic acid radical of strong acid. A part of sample gas from the filter 2 is suctioned by a pump 18 through pipelines 16, 14 on the way of a pipeline 3 to flow from a capillary 15 to a flow path 7 of the container 4 under lower pressure than that within a flow path 6 of the partition wall 5. Gas deprived of NH3 through the partition wall 5 passes from the flow path 6 to a flow meter 8, and then is absorbed into an absorbing solution 10 within an impinger 9. Conductivity of the absorbing solution 10 is measured by a gauge 11 and then recorded and displayed.