PURPOSE: To prepare the titled substance, in high purity, by crystallizing the substance from a 1,4,4a,9a-tetrahydroanthraquinone solution obtained from a 1,4- naphthoquinone solution and 1,3-butadiene, adding an enolizing catalyst to the filtrate, and treating the solution with NaOH.

CONSTITUTION: The gas produced by the catalytic vapor-phase oxidation reaction of naphthalene is scrubbed with water to obtain an aqueous slurry of 1,4- naphthoquinone (NQ) and phthalic acid. The slurry is extracted with a lower aromatic hydrocarbon to obtain an NQ solution, which is subjected to the Diels- Alder reaction with 1,3-butadiene to obtain 1,4,4a,9a-tetrahydroanthraquinone (THAQ) solution. THAQ is separated from the solution by crystallization, and the filtrate is enolized at 60W100°C in the presence of an enolizing catalyst. 1,4- Dihydroanthrahydroquinone separated by the process is dissolved in a aqueous solution of NaOH, and if necessary, oxidized with air to obtain anthraquinone.