Abstract: A catalytic vapor phase oxidation reactor which comprises a fixed-bed shell and tube heat exchange apparatus in which a bundle of a multiplicity of tubes filled with at least one type of oxidizing catalyst are disposed in a shell and these tubes are passed through the apertures formed in at least one perforated shield plate to partition the inside of the shell into at least two heat transfer medium feed zones and in such a manner that each of the tubes passing through the perforated shield plate is not in direct contaction with the shield plate but the outer surface of the tube and the inner surface of the aperture are spaced apart by a distance of between 0.2-5 mm, supplying feed gas to the tubes of the reactor, and conducting exothermic catalytic vapor phase oxidation while controlling the temperatures for the heat transfer medium in each of the zones so that the temperature difference between each of the zones can be maintained between 0.degree.-100.degree. C.