To reduce a waste treating cost of a dehydrated cake by a method wherein the dehydrated cake which is free from a fear of environmental disruption owing to dispersion of particles can be obtained.

An aggregate is recovered by separating from waste ready-mixed concrete discharged from a ready-mixed concrete plant, a concrete product plant, or a mixer car, and remaining slurry is stored in a stirring bath 30. The slurry in the stirring bath 30 is conducted to a shear bath 16. By rotating a vane 19 at a high speed, the cement in the slurry is sheared, and progress of coagulation by a hydrating operation is temporarily interrupted. Thereafter, the slurry is fed by pressure to a filter press 26, and dehydrated. A dehydrated cake 29 discharged from the filter press 26 is freezed in a massive state by continuation of the hydrating operation.