A process for removing an anthraquinone type scale comprises condensing a steam evaporated from a black liquor obtained by separating pulp from a digested mixture of lignocelluloses containing an anthraquinone type digesting assistant; and contacting a heated aqueous solution of a base in the presence or absence of a reducing agent or an alkali resistant surfactant with a wall of a condenser of an evaporator on which said anthraquinone type scale is adhered.