PROBLEM TO BE SOLVED: To remove water contained in a cyclohexanone oxime hydrochloride oily substance to be supplied to a Beckmann's rearrangement reaction. SOLUTION: Cyclohexanone oxime hydrochloride salt is mixed with an organic solvent (e.g. a halogenated hydrocarbon, chloroform, or 1,2- dichloroethane) poor in compatibility with water and large in compatibility with the cyclohexanone oxime hydrochloride and subsequently subjected to an azeotropic distillation treatment to separate and remove water contained in the salt.