A process for producing 2-cyclopropyl-4-(4-fluorophenyl)-quinoline-3-carbaldehyde important as intermediate for the synthesis of pharmaceuticals, efficiently from an unnecessary antipode, is provided.A process for producing 2-cyclopropyl-4-(4-fluorophenyl)-quinoline-3-carbaldehyde represented by the formula (III): ##STR00001## which comprises treating a compound represented by the formula (I) or (II): ##STR00002## (wherein A is --CHOH or --C(O)--, and R is a hydrogen atom, a C.sub.1-4 alkyl group which may be branched, a phenyl group, an alkali metal ion or an alkaline earth metal ion) with ozone, followed by reduction with an inorganic sulfur compound or by hydrogenation for reduction decomposition.