PURPOSE: To obtain the titled high-purity compound directly usable as an intermediate raw material for a dyestuff, by the oxidation of an addition product of 1,4- naphthoquinone with a conjugated diolefin in an aqueous caustic alkali under conditions at which the intermediate product may remain.

CONSTITUTION: Crude 1,4,4a,9a-tetrahydroanthraquinone (THAQ) formed by the Diels-Alder reaction of 1,4-naphthoquinone with a conjugated diolefin is added to a 10% aqueous caustic alkali containing a caustic alkali in a molar amount of 2.5 times, and dissolved under heating to convert the THAQ into an alkali metal salt of 1,4-dihydro-9,10-dihydroxyanthracene (DDA), in which molecular oxygen is blown until the alkali metal salt remains in a concentration of 0.5% or more, preferably 1W5%, as the DDA in the aqueous solution, and the reaction is stopped. The crystal of the anthraquinone thus formed is separated by the filtration, washed with water and dried.