PURPOSE: To obtain the subject compound with a high conversion and an efficient post treatment without the need of a large amount of an auxiliary agent by reacting acetaldehyde and ethanol in the presence of an acid catalyst and in the presence of an entrainer having a specific boiling point.

CONSTITUTION: The objective compound is obtained by reacting acetaldehyde with ethanol preferably with a 1:2 to 20 molar ratio in the presence of an acid catalyst and in the presence of an entrainer having a boiling point of 25 to 75°C, preferably 40 to 70°C (preferably a water-insoluble solvent, such as pentane, hexane, industrial hydrocarbon mixture, methylene chloride, diethyl ether, particularly preferable is hexane).