PURPOSE: To purify recovered -caprolactam into a highly pure product useful as a raw material for fiber in an industrially advantageous manner by purifying the recovered -caprolactam obtained by depolymerizing a thermoplastic resin containing nylon 6 as a main component.

CONSTITUTION: The method for purifying recovered -caprolactam comprises depolymerizing a thermoplastic resin containing nylon 6 as a main component [e.g. fibrous structures, fiber wastes. resin structures or resin wastes comprising nylon 6 in an amount of ≥50wt.%, cellulosic fibers or polyurethane in an amount of 0.1-30wt.%, or protein fibers, a polyacrylate, polyethylene terephthalate or nylon 66]. and subsequently purifying the recovered -caprolactam. The depolymerization is preferably performed by depolymerizing the component A in the presence of a phosphoric acid (salt) catalyst and water, and the purification is performed by adding 0.01-20wt.% of an alkali hydroxide to the recovered -caprolactam.