To obtain the subject highly pure compound useful as a rust-preventive agent by gradually adding the aqueous solution of a lower saturated aliphatic carboxylic acid to the aqueous solution of hydroxylamine under stirring in a reaction system maintained at a low temperature to react the hydroxylamine with the lower saturated aliphatic carboxylic acid.

This method for producing a highly pure lower saturated aliphatic carboxylic acid hydroxylamine comprises gradually adding a lower saturated aliphatic carboxylic acid (e.g. formic acid, acetic acid, propionic acid, butyric acid, oxalic acid) aqueous solution to a hydroxylamine aqueous solution or antrarily the hydroxylamine aqueous solution to the lower saturated aliphatic carboxylic acid in a reaction system maintained at ≤40°C under agitation preferably until giving a pH of 6.0 to the reaction system, thus reacting the hydroxylamine with the lower saturated aliphatic acid. When the crystals of e.g. oxalic acid hydroxylamine are separated from the reaction system, the oxalic acid hydroxylamine crystals produced just after the reaction can be separated from the reaction system, but the oxalic acid hydroxylamine crystals are preferably separated, after aged and grown.