This invention provides a method for producing acrolein and/or acrylic acid by catalytic gas-phase oxidation, which method makes it possible to carry out a continuous operation steadily for a long period of time while a high yield is maintained.This method is characterized by comprising filling each of reaction tubes of a fixed-bed multitubular reactor with at least two species of catalysts each of which essentially comprises, as catalytically active components, oxide of molybdenum, oxide of bismuth and oxide of iron and/or composite oxide of at least two of said elements, said at least two species of catalysts being different in the ratio of D1/D2, D1 denoting the proportion of the total pore volume of pores whose pore diameter falls within the range of at least 0.03 μm and less than 0.3 μm to the total pore volume of the whole pores, and D2 denoting the proportion of the total pore volume of pores whose pore diameter falls within the range of at least 0.3 μm and at most 3 μm to the total pore volume of the whole pores, in such a manner that at least two reaction zones are formed axially in each of the reaction tubes.