To provide an acidic gas treatment method by which in a feedback form for controlling additive amount of an alkali agent based on an acidic gas concentration measurement signal, failure in acidic gas treatment and excess addition of an alkali agent caused by measurement delay of present feedback control are reduced.

There is provided the acidic gas treatment method in which the alkali agent is added to acidic gas-containing exhaust combustion gas and the additive amount of the alkali agent is feedback-controlled on the basis of a measurement signal of the acidic gas concentration measuring instrument installed so as to measure acidic gas concentration after dust collection. The acidic gas treatment method includes the steps of: measuring concentration of the same kind of acidic gas by a HCl concentration measuring instrument (low speed) 14 and a HCl concentration measuring instrument (high speed) 15 which have measurement delay times different from each other; and calculating an output value of alkali agent additive amount by feedback calculation based on measurement signals of two or more acidic gas concentration measuring instruments.