PURPOSE: To reduce the amt. of discharged water and the amt. of treating solid matter in a wet desulfurization method using a Ca absorbing liquid by adding Na2CO3 and NaOH simultaneously or successively to the filtered water of desulfurization.

CONSTITUTION: Filtered water 112 of a desulfurizer which is the supernatant of a thickener separated of by-produced gypsum is conducted to a control tank 81 for filtered water where the softening treatment is performed. Na2CO3 17 and NaOH 800 are simultaneously added to the water there. Otherwise, a separate tank may be provided and NaOH 800 may be added afterward. The pH of the water is controlled to about ≥10 to settle heavy metals such as Mg2+ as a hydroxide and to decrease the concn. of Ca2+ considerably. The water is then conducted to a tank 82 for forming flocs where a floccuating agent 18 is added to the water to form flocs. The water is subjected to a solid-liquid sepn. in a floc settling thickener 83. Supernatant liquid 114 is stored as treated water in a tank 84, and is supplied through a line B to a desulfurizer. The thickened slurry 118 is fed to a waste water treatment device A and is thus treated.