A reaction tube of a reaction apparatus includes a circular cylindrical inner tube and an intermediate tube disposed concentrically with the inner tube. A catalyst used in reforming reactions is charged in the gap between the inner and the intermediate tubes. The inner tube accommodates filler particles and is provided with a hollow member disposed therein in contact with the filler particles and capable of absorbing stress resulting from thermal expansion of the filler particles. By virtue of the provision of the hollow member which absorbs thermal stress of the filler particles that are caused to undergo thermal expansion by a high-temperature gas, such as a combustion gas, flowing through the inside of the inner tube, the apparatus is capable of preventing any deformation or breakage of the inner tube.