A method for vapor phase catalytic oxidation for obtaining a reaction product gas by using a fixed bed multi-tube heat-exchanger type reactor provided with a plurality of reaction tubes, circulating a heating medium outside the reaction tubes, and feeding a raw material gas inside the reaction tubes packed with a catalyst, wherein the method comprises: predicting reaction states inside the reaction tubes; and changing catalyst packing specifications of the reaction tubes according to the prediction results so that nonuniformity of the reaction states among the reaction tubes are reduced, wherein the reaction states inside the reaction tubes are predicted by grasping thermal states inside the reaction tubes through a simulation analysis using a computer, and a fluid analysis of a heating medium is conducted through the simulation analysis using a computer, wherein the method further comprises: determining peak temperatures of the catalyst layers of the respective reaction tubes through the simulation analysis and an average value of the peak temperatures representing a whole reactor based on the results of the respective peak temperatures, comparing the average value of the peak temperatures and the peak temperatures of the respective reaction tubes, and changing the catalyst packing specifications for the reaction tubes having a temperature difference of 15°C or more with the average peak temperature, wherein the catalyst packing specifications are changed to reduce the temperatures inside the catalyst layers, wherein the catalyst packing specifications include items of a catalyst type, a catalyst amount, a catalyst shape, a dilution method for the catalyst, and lengths of reaction zones. predicting reaction states inside the reaction tubes; and changing catalyst packing specifications of the reaction tubes according to the prediction results so that nonuniformity of the reaction states among the reaction tubes are reduced, wherein the reaction states inside the reaction tubes are predicted by grasping thermal states inside the reaction tubes through a simulation analysis using a computer, and a fluid analysis of a heating medium is conducted through the simulation analysis using a computer, wherein the method further comprises: determining peak temperatures of the catalyst layers of the respective reaction tubes through the simulation analysis and an average value of the peak temperatures representing a whole reactor based on the results of the respective peak temperatures, comparing the average value of the peak temperatures and the peak temperatures of the respective reaction tubes, and changing the catalyst packing specifications for the reaction tubes having a temperature difference of 15°C or more with the average peak temperature, wherein the catalyst packing specifications are changed to reduce the temperatures inside the catalyst layers, determining peak temperatures of the catalyst layers of the respective reaction tubes through the simulation analysis and an average value of the peak temperatures representing a whole reactor based on the results of the respective peak temperatures, comparing the average value of the peak temperatures and the peak temperatures of the respective reaction tubes, and changing the catalyst packing specifications for the reaction tubes having a temperature difference of 15°C or more with the average peak temperature, wherein the catalyst packing specifications are changed to reduce the temperatures inside the catalyst layers, wherein the catalyst packing specifications include items of a catalyst type, a catalyst amount, a catalyst shape, a dilution method for the catalyst, and lengths of reaction zones.