PURPOSE: To prepare the titled compound economically in an industrial scale, by the catalytic vapor phase reaction of a gas containing benzene and phthalic acid anhydride, using a catalyst obtained by mixing a magnesia-silica mixture with other metal oxides and/or metal sulfates.

CONSTITUTION: The titled compound is prepared by the catalytic vapor phase reaction of a gas containing benzene and phthalic acid anhydride pref. at 350W 450°C using a catalyst obtained by mixing a magnesia-silica mixture with other metal oxides and/or metal sulfates. The heat necessary for the continuation of the reaction is pref. supplied by mixing the raw material gas with a heat medium comprising an inert gas such as CO2, N2, etc. The gas produced by the reaction is pref. introduced to a multitubular heat exchanger having small-sized tubes at a sufficiently low flow rate to prevent the scattering of the titled compound with the gas stream and to allow the deposition of the compound in the tube by cooling, and the deposited titled compound is separated from the heat-exchanger and collected.