Provided is a process in which cyclization of a compound having, in a molecule, a formyl group and a double bond which allow carbonyl-ene cyclization, the compound existing as a mixture of optical isomers thereof, increases the ratio of a particular optical isomer not only in a cyclized compound, but also in an unreacted compound. The process comprises performing, in the presence of a specified aluminum complex represented by the general formula: [Al2(L1)n(L2)3-m]m, cyclization of a compound having, in a molecule, a formyl group and a double bond which allow carbonyl-ene cyclization, the compound existing as a mixture of optical isomers thereof, to increase the ratio of a particular optical isomer.