To shorten the cooling time of an open oven after the temperature of the oven is raised by constituting the container main body of the oven so that air can get in and out of the main body and heating a separation column while the oven is opened.

The container main body 1 of an open oven is lined with heat insulating materials 2 and 14 and houses a separation column 4, a heating heater 6, and a fan 7. At the same time, the main body 1 is constituted so that air can get in and out of the main body 1 and the column 4 can be heated while the main body 1 is opened. In addition, the clearance between a lid body 12 and the opening 1a of the main body 1 is made larger so that the outside air can be quickly taken in and discharged to the outside from the main body 1 with the fan 7. Since the column 4 can be heated while the outside air is introduced, the temperature rises of the insulating materials 2 and 14 can be suppressed and the cooling time of the materials 2 and 14 can be shortened. Therefore, the analyzing cycle time of the open oven can be shortened and the efficiency of continuous measurement can be improved.