Abstract: An oxidation apparatus for applying oxidation treatment to a plurality of semiconductor wafers includes a vertical-type reaction tube in which the wafers are housed, and an outer tube coaxially located outside the reaction tube with a space interposed between them. A heater is located enclosing the outer tube. The space between both of the tubes serves as a passage for pre-heating a treating gas passing through the space. A pipe for supplying the treating gas into the space is connected to the lower portion of the outer tube. A diffusion plate is formed at the top of the reaction tube and it is provided with a plurality of diffusion holes through which the treating gas is introduced into the reaction tube after the gas passes through the space. The space between both of the tubes is partitioned by a spiral pipe made of quartz and the gas pre-heating passage is thus formed spiral extending from the gas supply pipe to the diffusion plate.