PURPOSE: To obtain phthalic anhydride advantageously, by bringing a molecular oxygen-containing gas mixture having indene, etc., into contact with a catalytic layer prepared by supporting each specific amount of titanium dioxide, vanadium pentoxide and an alkali metallic compound in a nonporous inert carrier.

CONSTITUTION: A molecular oxygen-containing gas mixture having indene and/or indane and preferably further naphthalene and/or o-xylene is brought into contact with a catalytic layer packed with a catalyst prepared by supporting a catalytic active component consisting of 70W90wt.% titanium dioxide, 10W30wt.% vanadium pentoxide and 0.5W5wt.% (calculated as sulfate) alkali metallic compound on a nonporous inert carrier and oxidized to give phthalic anhydride in high yield and high productivity by using indene and/or indane relatively richly contained in coal tar or petroleum fraction as part or the whole of the raw material.