This invention relates to a method for the treatment of a desulfurization absorbing fluid after gas-liquid contact with combustion exhaust gas in which fine particles of unburned carbon and other materials are efficiently removed from the desulfurization absorbing fluid to improve the quality of a by-product (e.g., gypsum) obtained therefrom. The method of this invention comprises the steps of feeding a gas into a desulfurization absorbing fluid (A) containing fine particles of unburned carbon and other materials and thereby producing gas bubbles so as to cause the fine particles to adhere to the surfaces of the gas bubbles and create a foam phase (D) consisting of the gas bubbles; and breaking the foam phase (D) to obtain a liquid (E) containing the fine particles.