PURPOSE: To simplify a gypsum recovery and liquid treatment system to a large extent to reduce installation cost and operation cost by using a gypsum separator without using a gypsum thickener in the recovery of gypsum and treating the solid in a filtrate in a fluorine treatment sedimentation tank.

CONSTITUTION: In a first step, the gupsum slurry from a desulfurization gas absorbing tower 1 is supplied to a gypsum separator 11 to be separated into gypsum and a filtrate and, in a second step, the filtrate separated in the first process is supplied to a fluorine treatment sedimentation tank 18 for waste water treatment to be separated into an impure solid and a supernatant soln. In a third step, the impure solid separated in the second step is supplied to the separator 11 along with gypsum in the first step to be recovered along with gypsum. As a result, a gypsum recovery and liquid treatment system is simplified to a large extent to reduce installation cost and operation cost.