To obtain the subject compound useful as a curing agent for rubber, capable of readily converting a halomethyl group having relatively large steric hindrance into an aldehyde functional group in a high yield without using a chlorinated solvent or hexamethylenetetramine.

This method for synthesizing a stable nitrile oxide compound comprises the steps of halomethylating (A) a substituted aromatic compound containing a substituent group such as an alkyl group or the like to form (B) an orthohalomethylated-aromatic compound, reacting the component B with a salt selected from the group consisting of sodium 2-nitropropane and potassium 2-nitropropane to convert the component into (C) an ortho-substituted aromatic aldehyde, reacting the component C with hydroxylamine to convert the component into (D) an ortho-substituted aromatic oxide and reacting the component D with an aqueous solution of sodium hypochlorite at about -5 to about 20°C to covert the component into (E) an ortho-substituted arylnitrile oxide.