A laboratory-scale apparatus for manufacturing an absorbent body, comprising: a conveying unit that conveys an aggregate of absorbent fibers and a water-absorbent resin; a mixing unit that accepts the aggregate of the absorbent fibers and the water-absorbent resin that were conveyed by the conveying unit and mixes the absorbent fibers and the water-absorbent resin; a chamber portion that has an internal space through which the absorbent fibers and the water-absorbent resin that were mixed in the mixing unit fall; a stacking portion that is disposed below the chamber portion and has a recessed portion in which the absorbent fibers and the water-absorbent resin that fall through the chamber portion are stacked; and a suction unit that causes a negative pressure in the chamber portion, wherein the mixing unit includes: a cylindrical body that is supported rotatably and has an outer circumferential surface on which recesses and protrusions are formed; a driving portion that rotates the cylindrical body; and a gap formation portion that includes a pair of circular frames that are disposed outward from both sides of the cylindrical body and whose outer diameter is slightly larger than that of the cylindrical body, wherein a gap is formed between a cover member and a mesh member attached on these frames and the outer circumferential surface of the cylindrical body, the aggregate of the absorbent fibers and the water-absorbent resin that were conveyed by the conveying unit pass through the gap, and the gap is smaller than a thickness of the aggregate of the absorbent fibers, and the gap formation portion includes: a connection portion that is located below the cylindrical body, is provided with a plurality of through holes, and connects the gap and the internal space of the chamber portion via the plurality of through holes; and a mixing portion that is disposed on at least an upstream side of the connection portion and is provided with recesses and protrusions facing the outer circumferential surface of the cylindrical body. a conveying unit that conveys an aggregate of absorbent fibers and a water-absorbent resin; a mixing unit that accepts the aggregate of the absorbent fibers and the water-absorbent resin that were conveyed by the conveying unit and mixes the absorbent fibers and the water-absorbent resin; a chamber portion that has an internal space through which the absorbent fibers and the water-absorbent resin that were mixed in the mixing unit fall; a stacking portion that is disposed below the chamber portion and has a recessed portion in which the absorbent fibers and the water-absorbent resin that fall through the chamber portion are stacked; and a suction unit that causes a negative pressure in the chamber portion, wherein the mixing unit includes: a cylindrical body that is supported rotatably and has an outer circumferential surface on which recesses and protrusions are formed; a driving portion that rotates the cylindrical body; and a gap formation portion that includes a pair of circular frames that are disposed outward from both sides of the cylindrical body and whose outer diameter is slightly larger than that of the cylindrical body, wherein a gap is formed between a cover member and a mesh member attached on these frames and the outer circumferential surface of the cylindrical body, the aggregate of the absorbent fibers and the water-absorbent resin that were conveyed by the conveying unit pass through the gap, and the gap is smaller than a thickness of the aggregate of the absorbent fibers, and the gap formation portion includes: a connection portion that is located below the cylindrical body, is provided with a plurality of through holes, and connects the gap and the internal space of the chamber portion via the plurality of through holes; and a mixing portion that is disposed on at least an upstream side of the connection portion and is provided with recesses and protrusions facing the outer circumferential surface of the cylindrical body. a cylindrical body that is supported rotatably and has an outer circumferential surface on which recesses and protrusions are formed; a driving portion that rotates the cylindrical body; and a gap formation portion that includes a pair of circular frames that are disposed outward from both sides of the cylindrical body and whose outer diameter is slightly larger than that of the cylindrical body, wherein a gap is formed between a cover member and a mesh member attached on these frames and the outer circumferential surface of the cylindrical body, the aggregate of the absorbent fibers and the water-absorbent resin that were conveyed by the conveying unit pass through the gap, and the gap is smaller than a thickness of the aggregate of the absorbent fibers, and the gap formation portion includes: a connection portion that is located below the cylindrical body, is provided with a plurality of through holes, and connects the gap and the internal space of the chamber portion via the plurality of through holes; and a mixing portion that is disposed on at least an upstream side of the connection portion and is provided with recesses and protrusions facing the outer circumferential surface of the cylindrical body. a connection portion that is located below the cylindrical body, is provided with a plurality of through holes, and connects the gap and the internal space of the chamber portion via the plurality of through holes; and a mixing portion that is disposed on at least an upstream side of the connection portion and is provided with recesses and protrusions facing the outer circumferential surface of the cylindrical body.