To prevent the generation of electrolytic bubbles in the fused glass in the spacing between a stirrer and a stirrer tube by avoiding the flow of DC current to the fused glass in the spacing between the stirrer and the stirrer tube.

A potential regulating means 52 sets the potential of an AC power source 50 for heating at the min. in the position of the stirrer 36 to suppress the electromotive force formed in the fused glass G in the spacing between the 36 stirrer and the stirrer tube 31. A DC power source 66 erases the electromotive force formed at the time of stirring the fused glass G in the stirrer tube 31 by rotating the stirrer 36 and the electromotive force suppressed by this potential regulating means 52. As a result, the flow of the DC electric power to the fused glass G is prevented.