Disclosed is a method of a gas-phase catalytic oxidation reaction of propylene, isobutylene, or tertiary butanol with molecular oxygen in the presence of a catalyst to produce a corresponding unsaturated aldehyde and a corresponding unsaturated carboxylic acid, in which the catalyst can be used over a long period of time. Concretely, in the presence of the catalyst containing a complex oxide including molybdenum, bismuth and iron as essential components, at least one factor of a reaction pressure and a molar ratio of molecular oxygen to a raw material is controlled to change in such a way that a rate of reaction of the raw material is kept constant in the temperature range of from (TA-15)° C. to TA° C., when a boundary temperature of the activation energy of the catalyst is set to be TA° C.