PURPOSE: To inexpensively and easily prepare atmospheric gas for heat treatment, by mixing waste oils or saccharides with air, and circulating the gaseous mixture through a high-temp. carbon layer to decompose said oils or saccharides.

CONSTITUTION: An atmospheric gas-forming apparatus constituted by a hermetically sealed tubular heat-insulating material 1 is packed with carbon particles to form a carbon layer 2. Upper and lower electrodes 3 and 4, each comprising a couple of electrodes facing to each other, are provided therein. By impressing voltage on the electrodes, an electric current is directly applied to the carbon layer to heat it. In addition, air having a flow rate regulated by a gas flow meter 12 is sent into the carbon layer 2, while fats, oils or saccharides, e.g. waste oils or saccharides, or raw materials, e.g. macromolecular compounds are sufficiently mixed with retained water by a heater 8 and an agitator 9 inside a combustible material tank 7, sent through a liquid flow meter 11 to the heated carbon layer 2 by feeding the liquid under pressure through a pressurizing opening 9 by a press for pressure feeding, and decomposed in the carbon layer 2. The obtd. atmospheric gas is sent through an opening 13 for withdrawing formed atmospheric gas to an oven for heat treatment.