To efficiently obtain decolored ammonium sulfate by extracting a part of a mother liquor in a crystallization kettle, decomposing the contained org. matter by oxidation to remove the org. matter and then returning the mother liquor to the kettle when an ammonium sulfate soln. formed in the production of lactam by the Beckmann rearrangement reaction is charged into the kettle and concentrated to deposit crystallized ammonium sulfate. The rearranged ammonium sulfate soln. formed in the production of lactam by the Beckmann rearrangement reaction is supplied into the ammonium sulfate crystallization kettle 1 and/or 2, and concentrated at 30 to 150 deg.C under reduced pressure to deposit crystallized ammonium sulfate. In this case, a part of the mother liquor having a low content of crystallized ammonium sulfate is extracted from the upper part of the kettle 1 or 2, pressurized to >=20kg/cm<2>, transferred to the oxidizing and decomposing tower 4, and brought into contact with a molecular oxygen-contg. gas at 200 to 300 deg.C and 10 to 100kg/cm<2> to decompose the contained org. matter by oxidation. The high-temp. gaseous component is discharged from the top of the tower 4. The mother liquor with the contained org. matter reduced is depressurized, cooled, circulated to the kettle 1 or 2, concentrated, and then discharged from the bottom. The deposited crystallized ammonium sulfate is recovered by centrifugal separation. |