Disclosed is an improved process for producing nitroguanidine derivatives represented by the formula (3), which has an insecticidal activity, or a salt thereof, wherein the substituents are defined herein, in which nitroisourea derivatives represented by the following general formula (1) or a salt thereof wherein the substituents are defined herein, and compounds represented by the following general formula (2) or a salt thereof, wherein the substituents are defined herein, are reacted in the presence of a base in an aqueous solution wherein an inorganic salt is dissolved at not less than 50% of its saturated solubility.