PURPOSE: To efficiently recover the titled compound while preventing blocking of an apparatus by a residue, by feeding the distillation residue of crude phthalic anhydride produced by oxidation of naphthalene into a kneader-extruder, volatilizing a remaining phthalic anhydride under reduced pressure to a specific concentration and extruding the residue at a low viscosity.

CONSTITUTION: A distillation residue of crude phthalic anhydride prepared by oxidizing naphthalene is continuously fed to an inlet nozzle 7 of a kneader- extruder 6 from a tank 2 with a pump 4 and the residue is passed through the kneader-extruder 6 by rotation of screws 62 and 62 while heated with a heating medium in a heat transfer jacket tube 64 whereby volatilizing the phthalic anhydride contained in the distillation residue from a degassing nozzle 8 by suction of a pressure reducing apparatus and recovering by condensation with a condenser 13 into a tank 16, whereby the concentration of the remaining phthalic anhydride is adjusted to ≤5wt% and the distillation residue is discharged from a discharge nozzle 11.