A hydroquinone diester derivative represented by the formula (1): wherein R1 and R2 are the same or different, each representing an alkyl group, a cycloalkyl group, an aryl group, or a heterocyclic group is crystallized from a reaction mixture containing the hydroquinone diester derivative, and the resulting product in crystallized form is washed. A solvent for the crystallization is composed of an organic carboxylic acid and water, and the crystallized product is washed with warm or hot water of 40° C. or above. In the formula (1), R1 and R2 are C1-4alkyl groups.