To prevent heat from being dissipated from the inside of a furnace and to enhance the accuracy of a analysis by forming a furnace body to have a two-layer structure in which a transparent quartz glass layer and an opaque quartz glass layer are formed sequentially from the side of the inside of a U-shaped space.

Sample mounting containers 2, on which samples S to be analyzed are mounted, are placed on a sample mounting base 3. A baseplate 4 which supports the sample mounting base 3 can be raised and lowered by a raising and lowering member 5. When the samples S are treated, the raising and lowering member 5 is raised to the uppermost position, and an inside closed space region 7 is formed between the lower end part of a furnace body 6 and the baseplate 4. In addition, a nozzle 8 is passed through the baseplate 4 and the inside of the raising and lowering member 5, one end of it is opened in the inside closed space region 7, and a gas is supplied. At this time, the furnace body 5 6 is made of quartz glass, and it is formed to be of a two-layer structure whose inside is formed of a transparent layer and whose outside is formed of an opaque layer. At this time, it is preferable that the thickness of the transparent layer is at 10 to 30% with reference to the total thickness of the furnace body 6 and that the thickness of the opaque layer is at 70 tp 90%. Thereby, the parquet layer prevents heat from being dissipated from the inside of a furnace, the uniform heating property at the inside of the furnace is kept, and th accuracy of an analysis can be enhanced.