PURPOSE: To obtain the title compound useful as an assistant for the preparation of hydrogen peroxide, etc., in high yield, by the ring closure of alkylbenzoylbenzoic acid, using a specific ring-closing agent and selecting the reaction temperature and the reaction time.

CONSTITUTION: Alkylanthraquinone is obtained by the reaction of alkylbenzoylbenzoic acid (e.g. mixed amylbenzene, etc.) of the formula (R is 2W8C alkyl) in the presence of a ring-closing agent. In this occasion, concentrated sulfuric acid of more than 98wt% or oleum containing less than 10wt% of SO3 is used as the ring-closing agent. The reaction temperature is maintained at 100W150°C, and the reaction time is less than 60min.

EFFECT: The amount of the ring-closing agent can be reduced to 1/2W1/3 of the conventional process, and the amount of waste acid can also be reduced.