An apparatus for applying surface treatment on an object-to-be-treated which is continuously moved in a treating liquid usually by a hanging conveyor. The apparatus is provided at the bottom portion of a treating vessel with one or more hoppers, which function to collect the deposited sludge for exhausting the same through the lower portion of the hoppers outside the treating vessel. Further, the apparatus is provided with a plurality of slant plates disposed above the bottom of the treating vessel, being inclined in the same direction, with a predetermined inter-distance among them, so as to facilitate sludge produced in the vessel to be gradually deposited down on the bottom thereof.