Disclosed is a niobium(Nb)-containing aqueous solution for use in producing a Nb-containing oxide catalyst, wherein the oxide catalyst comprises an oxide of a plurality of active component elements including Nb and is for use in a catalytic oxidation or ammoxidation of propane or isobutane in the gaseous phase, and wherein the oxide catalyst is prepared by a process comprising mixing the Nb-containing aqueous solution with (an) aqueous mixture(s) containing compounds of active component elements of the oxide catalyst other than Nb, to thereby provide an aqueous compound mixture, and drying the aqueous compound mixture, followed by calcination. The Nb-containing aqueous solution comprises water having dissolved therein a dicarboxylic acid, an Nb compound and optionally ammonia, wherein the dicarboxylic acid/Nb molar ratio (α) satisfies: 1 ≦ (α) ≦ 4, and the ammonia/Nb molar ratio (β) satisfies: 0 ≦ (β) ≦ 2. By the use of the niobium-containing aqueous solution, it has become possible to produce efficiently an oxide catalyst which, when used in a catalytic oxidation or ammoxidation of propane or isobutane in the gaseous phase, exhibits high catalytic activity, and which, therefore, can be advantageously used for producing (meth)acrylic acid or (meth)acrylonitrile in high yield.
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