PURPOSE: To reduce weak bondings in an oxidized film, to reduce the material to be trapped in the film, and to improve reliability of such as an MOS transistor by forming a thermally oxidized film on the surface of a material by irradiating the surface with ultraviolet rays.

CONSTITUTION: A heating device 102 is provided to the outside periphery of a cylindrical hollow quartz tube 101, and a window for irradiating ultraviolet rays is formed on a part of the quartz tube for the purpose of irradiating a material 104 to be oxidized in the quartz tube with ultraviolet rays. A source 105 of the ultraviolet rays, reflector 106, filter 107 for far ultraviolet radiation, and a filter 108 for infrared radiation are provided to above the window. By this device, weak bondings in the oxidized film are reduced, and the amt. to be trapped in the film is reduced. Thus, the reliability of such as an MOS transistor is improved.