PURPOSE: To make possible measurement of wet concentration (concentration in an atmosphere containing steam) and/or dry concentration concentration in an atmosphere containing no steam) of a substance to be measured using one environmental atmosphere analyzer, e.g. a sulfur dioxide analyzer.

CONSTITUTION: A steam quantity measuring section 7 is disposed in parallel with a gas sucking section 4 and an operational processing section 10 is connected with the gas sucking section 4 and the steam quantity measuring section 7. The steam quantity measuring section 7 measures relative humidity H, temperature and pressure Pw of a sample atmosphere whereas the gas sucking section 4 measures wet concentration Cw of sulfur dioxide in the sample atmosphere. The operational processing section 10 operates the pressure of steam Ps=Ps. max*H/100 using Ps.max determined by the relative humidity H and temperature. Dry concentration Cd is then operated according to a formula Cd=Cw*Pw/(Pw-Ps) using Ps, Pw and Cw.