An object of the present invention is to provide a catalytic gas phase oxidation reaction in which: even under reaction conditions of a higher gas pressure, a higher concentration of the raw material gas and a larger space velocity of a reaction gas, the thermal accumulation at the hot spot portion can be sufficiently suppressed with ease and at low costs, so that the reaction can be continued for a long time while a high yield is maintained. As a means of achieving this object, a catalytic gas phase oxidation reaction according to the present invention is a catalytic gas phase oxidation reaction with molecular oxygen or a molecular-oxygen-containing gas by using a fixed-bed multitubular reactor packed with catalysts; with the catalytic gas phase oxidation reaction being characterized in that: a catalyst-packed layer of each reaction tube of the reactor is divided into at least two reaction zones in a tubular axial direction; and the packing of the catalysts is such that the occupation volumes of the catalysts differ between at least two of the reaction zones and that an inert substance molding is mixed in at least one of the reaction zones.