The present invention relates to a multitubular catalytic gas-phase reaction apparatus, comprising a plurality of reaction tubes arranged in parallel, a baffle capable of changing the direction of movement of a heat medium introduced into a reactor shell to a direction perpendicular to the longitudinal direction of the reaction tubes, a space that is disposed in a region including a part of a section where the heat medium flows in the direction of the face of the baffle and does not have the reaction tubes arranged therein, and flow-adjusting rods, disposed in between the space and the reaction tubes, having the same longitudinal direction as that of the reaction tubes.