PURPOSE: To stably obtain the compound useful as an intermediate raw material for various organic compounds used as medicines, monomers for polymers, etc., in a high yield for a long period by cyclizing 5-(o-tolyl)-2-pentene under specific conditions.

CONSTITUTION: 5-(O-Tolyl)-2-pentene stored in the atmosphere having an oxygen concentration of ≤100ppm or under a light-shut condition and/or under a condition of ≤500°C in an atmosphere having an oxygen concentration of ≤100ppm as a raw material is subjected to a cyclization reaction in the presence of a catalyst (e.g. crystalline aluminosilicate-silicon oxide) in a diluent (e.g. nitrogen) at 100-400°C to obtain 1,5-dimethyltetralin. The strict control of the storage state of the raw material enables to advance the reaction without producing tar on the catalyst and without lowering the performances of the catalyst.