Urea derivatives of the formula ##STR1## (wherein Ar represents a residue of 5- or 6-membered heteroaromatic ring; R represents a hydrogen atom or a C.sub.1 -C.sub.6 alkyl group; R.sup.1 represents a C.sub.1 -C.sub.6 alkyl group; and R.sup.2 represents a C.sub.1 -C.sub.6 alkyl, C.sub.2 -C.sub.6 alkenyl, or C.sub.1 -C.sub.6 alkoxy group) are produced by reacting an amine of the formula Ar--NH--R (wherein Ar and R have the significance given above) with a carbamoyl halogenide of the formula ##STR2## (wherein X represents a halogen atom, and R.sup.1 and R.sup.2 have the significance given above) in the presence of a Lewis acid in an inert solvent.