PURPOSE: To smooth heating and to remove efficiently the steam from a sample by punching holes to a heating plate to be heated by a radiation heater, covering the bottom thereof with a perforated plate providing a temp. detecting end to the heating plate.

CONSTITUTION: Holes 11a are punched to the part of a heating plate 11 on which a decomposing vessel 1 is placed and the bottom is sealed with a perforated plate 11b with a heater of this decomposing device. The plate 11 and the vessel 1 are directly heated by radiation and at the same time the heating by the heat transfer from the plate 11 is accomplished. Indirect control of the temp. in the vessel 1 is thus made possible by controlling the surface temp. of the plate 11. It is possible to control simultaneously the temp. of plural samples under decomposition by controlling the temp. of the plate 11. The construction of the device is made considerably simpler than the heating device of the conventional method which detects the temp. of each sample and controls the respective heaters.