A recovery device (2, 103) configured to specifically extract as a dewatering aid hardly decomposable organic matter primarily composed of plant-derived fibrous matter from sludge generated in an earlier stage of a sewage treatment plant, wherein the recovery device (2, 103) comprises: a grinder configured to continuously grind and fragment fiber components to lengths within a range of recovery and granular solids and to untangle the fiber components and the granular solids to obtain prepared sludge; a separator configured to separate from the prepared sludge the fiber components from small foreign substances having sizes not more than a lower limit of the range of recovery; and an extractor configured to continuously extract from the separator the fiber components having specific properties separated by the separator so as to obtain said dewatering aid, wherein the hardly decomposable organic matter having the specific properties is fiber components having fiber lengths of 0.1 mm to 5 mm and fiber diameters of 1 μm to 50 μm, wherein the separator comprises: a rotatable cylindrical screen; a plurality of rolling elements mixed into the cylindrical screen and configured to roll while being transported; a rolling element tank configured to recover the rolling elements; and a return unit configured to return the rolling elements recovered in the rolling element tank to a supply portion of the separator from the rolling element tank through a circulation pipe, and wherein the extractor comprises a helically wound screw impeller arranged in the cylindrical screen. a grinder configured to continuously grind and fragment fiber components to lengths within a range of recovery and granular solids and to untangle the fiber components and the granular solids to obtain prepared sludge; a separator configured to separate from the prepared sludge the fiber components from small foreign substances having sizes not more than a lower limit of the range of recovery; and an extractor configured to continuously extract from the separator the fiber components having specific properties separated by the separator so as to obtain said dewatering aid, wherein the hardly decomposable organic matter having the specific properties is fiber components having fiber lengths of 0.1 mm to 5 mm and fiber diameters of 1 μm to 50 μm, wherein the separator comprises: a rotatable cylindrical screen; a plurality of rolling elements mixed into the cylindrical screen and configured to roll while being transported; a rolling element tank configured to recover the rolling elements; and a return unit configured to return the rolling elements recovered in the rolling element tank to a supply portion of the separator from the rolling element tank through a circulation pipe, and a rotatable cylindrical screen; a plurality of rolling elements mixed into the cylindrical screen and configured to roll while being transported; a rolling element tank configured to recover the rolling elements; and a return unit configured to return the rolling elements recovered in the rolling element tank to a supply portion of the separator from the rolling element tank through a circulation pipe, and wherein the extractor comprises a helically wound screw impeller arranged in the cylindrical screen.