PURPOSE: To easily pass the light from visible to near infrared range and to eliminate a disadvantage of insufficient heat insulating effect by using ceramics and fiber in combination as a heat insulator.

CONSTITUTION: Silicon carbide, silicon nitride, aluminum oxide, zirconium oxide, etc., for example, employed as ceramics of a heat insulator. Then, silica alumina, quartz, alumina, zirconia, etc., are employed, for example, as a fiber of another heat insulator. When quartz fiber out of them is employed, heat insulation is particularly satisfactory, and transfer of impurity to a quartz tube to be heat insulated is not substantially detected giving a particularly desirable result. As an actual heat insulating method for the tube, ceramics are provided on the part to be heat insulated such as a device of the surface of the tube such as a quartz reaction tube is exposed, and a fiber is provided thereon in a two- layer structure, or a multilayer structure of the repetition thereof is employed to mostly enhance the heat insulating effect satisfactorily.