Abstract: In the gas phase oxidation of an organic compound, using a metallic oxide fluidized-bed catalyst that contains molybdenum in an amount of 10% by weight or more, bismuth, and at least an element selected from the group consisting of iron and cerium, when tellurium is incorporated into this catalyst in such an amount that the atomic ratio of the tellurium to the molybdenum is (0.05-1.5):10, the catalytic activity can be retained for a prolonged period of time, and, at the same time, the loss of molybdenum and tellurium can be prevented during the oxidation reaction.