PURPOSE: To recover EP ash consisting essentially of Glauber's salt by adding a salt solution containing sodium chloride or sodium sulfate to EP ash containing an alkali metal salt, slurrying, eluting a potassium component and separating the resulting substance into a solid substance and a salt solution.

CONSTITUTION: (A) EP ash containing an alkali metal salt formed by a recovery boiler is blended with (B) a salt solution containing 1-2.5 times as much sodium chloride or/and sodium sulfate as the EP ash by weight into a slurry state, a potassium component contained in the EP ash of component is eluted, the resulting substance is separated into a solid substance and a salt solution and EP ash consisting essentially of Glauber's salt is obtained to recover Glauber's salt from the alkali metal salt-containing ash.