PURPOSE: The reaction mixture gas after thermal catalysis between benzene and phthalic anhydride is passed through a tubular type cooler at such a flow rate as the precipitating particles do not stick to the tube walls and brought into contact with benzene in a scrubbing dust collector to obtain the titled substance from the solution or slurry.

CONSTITUTION: The reaction mixture gas from the reactor 1 is introduced into the tubular cooler 2 where fine particles of anthraquinone and phthalic anhydride are precipitated in order, then fine liquid drops of benzene are condensed in the tubular cooler 3. At this time, a gas is fed to flow at such a speed as these precipitated particles and liquid drops do not stick on the tube walls. The gas containing these praticles is led to the scrubbing dust collector 4 where the anthraquinone and phthalic anhydride are partially dissolved in cold benzene and the rest forms a slurry. A part of the slurry is sent to the benzene evaporator 8 to obtain anthraquinone. Fresh benzene is sent from the storage tank 11. When the tube walls of the cooler 2 are scaled, the operation is switched to another cooler 2' and a heated inert gas is passed through the cooler 2.