Vanadium-containing catalysts are obtained by using polyvanadic acid as a source of vanadium. Vanadium-containing catalysts are obtained by mixing catalyst components other than vanadium, or their precursors, with a polyvanadic acid sol which is formed by ion-exchanging a metavanadic acid aqueous solution with a proton-type cation-exchange resin and performing polycondensation, and by drying and/or calcining the mixture. Such vanadium-containing catalysts can fully exhibit their catalytic activity under mild reaction conditions, and can be suitably used for various reactions, such as synthesis of phthalic anhydride by the partial oxidation of o-xylene, synthesis of benzaldehyde by the partial oxidation of toluene, synthesis of benzoic acid by the partial oxidation of toluene, synthesis of anisaldehyde by the partial oxidation of p-methoxy toluene, synthesis of propylene by the oxidative dehydrogenation of propane, synthesis of isobutene by the oxidative dehydrogenation of isobutane, synthesis of methyl formate by the oxidative dehydrogenation of methanol, and synthesis of acrylonitrile by the ammoxidation of propane.