To efficiently dispose of and recover a used paper diaper, and the like, by water treatment.

Water treatment work and grinding treatment work are applied to a paper diaper to grind solid matter into finely out pieces and to make them into a turbid water and bulkiness of a polymer absorbent being the solid matter for treatment is made into finely cut pieces and, therefore, water content of the polymer absorbent being finely cut pieces is forcibly separated, and separated water content, the polymer absorbent and solid residue other than the polymer absorbent are separated and recovered.