A vanadium-containing catalyst produced by using polyvanadic acid as the vanadium source. Such a catalyst can be produced by mixing a polyvanadic acid sol prepared by subjecting an aqueous solution of a metavanadate salt to ion exchange with a proton-form cation-exchange resin and polycondensation with the catalyst-constituting components other than vanadium or precursors thereof, and drying and/or firing the obtained mixture. This catalyst exhibits enough catalytic activity even under mild reaction conditions and can suitably be used in the synthesis of phthalic anhydride by partial oxidation of o-xylene, that of benzaldehyde by partial oxidation of toluene, that of benzoic acid by partial oxidation of toluene, that of anisaldehyde by partial oxidation of p-methoxytoluene, that of propylene by oxidative dehydrogenation of propane, that of isobutene by oxidative dehydrogenation of isobutane, that of methyl formate by oxidative dehydrogenation of methanol, that of acrylonitrile by ammoxidation of propane, and so on.