A mixture comprising 500 mg of 4-chloro-5-(3-n-propoxy-4-methoxybenzylamino)-6-nitro-3(2H)pyridazinone (Compound No. 22) prepared in Example 2, 691 mg of isopropyl iodide, 562 mg of anhydrous potassium carbonate and 25 ml of methyl ethyl ketone, was refluxed under stirring for 1.5 hours. The solvent was distilled off under reduced pressure, and water was added to the residue thereby obtained. The mixture was extracted with diethyl ether. The extract was washed with a saturated sodium chloride aqueous solution and dried over sodium sulfate. Then, the solvent was distilled off. The residual oily substance thereby obtained was crystallized from a solvent mixture of diethyl ether-n-hexane, to obtain 435 mg of the above identified compound having a melting point of from 82.5° to 84° C. as yellow crystals.
