To eliminate the generation of industrial waste in a ready mixed- concrete plant by solidifying/crushing/recycling so-called cake obtained by dewatering wastewater discharged in cleaning a mixer carriage.

In a closed apparatus for treating industrial waste, a cleaning apparatus for cleaning ready mixed-concrete adherent to the mixer carriage. a separator for separating sand and gravel from the wastewater discharged from the cleaning apparatus a dewaterer for removing water in the wastewater discharged from the separator, and a crusher for crushing a mixture comprising solids which were dewatered by the dewaterer and solidified and the solidified returned ready mixed-concrete are provided, and a material crushed by the crusher is recycled as a roadbed material, or the like.