The present invention relates to a method for producing an isoindole derivative (compound (II)) with the following general formula (II): ##STR00001## (wherein R.sup.1 and R.sup.2 each independently represents a C.sub.1-6 alkyl group) or a salt thereof, comprising the step of cyclizing, in a solvent, compound (I) with the following general formula (I): ##STR00002## (wherein R.sup.1 and R.sup.2 have the same meanings as R.sup.1 and R.sup.2 in formula (II) above) or a salt thereof, or their hydrate or solvate in the presence of a base (Step 1).