PURPOSE: To elevate the temp. of the whole glass raw material in a preheating device to a high temp. and to improve thermal efficiency of a glass melting furnace by maintaining the temp. of a part of the melting furnace where melt-sticking of the glass raw material is feared at an appropriate temp. by a cooling device.

CONSTITUTION: When briquettes as the glass raw material are fed to an internal cylinder 13, a briquette layer 10 is formed and moved downward. Waste gas at high temp. is fed to the top of an external cylinder, and flows from the external cylinder 16 to the internal cylinder 13 along arrow marks 11 to preheat the briquette layer 10. On one hand, an annular duct 14 is provided to the neighbourhood where the internal cylinder 13 connects with the external cylinder 16, and air is fed from its air feeding port 14a to the external cylinder 16. The air flows along the inside surface at the bottom of the internal cylinder 13 as shown by the arrow marks 15, and mixes with the high temp. waste gas, and then is discharged. Accordingly, the briquettes at the bottom of the internal cylinder 13 and the neighborhood are cooled weakly and cooling to ≤800°C is prevented.