Conventional gas-phase catalytic oxidation reaction apparatus comprising two reactors for production of acrylic acid is subject to problems of high equipment costs for the reactors and piping, necessity for wide installation area and easy accumulation of carbides generated by autoxidation of acrolein. On the other hand, conventional reaction apparatus comprising a single reactor has the drawbacks that the composition of gas in the first stage reaction and in the second stage reaction cannot be optimized independently of each other, and that the allowable concentration of starting propylene is limited because of the risk of explosion. As a reaction apparatus to solve these problems, this invention provides a fixed bed shell-and-tube reaction apparatus comprising a single reactor, characterized in that the inside of the reactor is divided into two reaction zones of the first reaction zone and the second reaction zone, and that a space equipped with a mechanism for introducing a gaseous substance from outside is provided between the two reaction zones.