PURPOSE: To detect the filament breakage, instantaneously, without using particular apparatus, by attaching a temperature sensor to the bottom of a bushing, and setting a signaling device so as to generate a signal when the rate of temperature increase at the bushing exceeds a specific level.

CONSTITUTION: A temperature sensor is attached to the bottom of a bushing, and the electric current to heat the bushing is controlled by a controller according to the bushing temperature. When a filament breakage extends to an extent to necessitate the interruption of the operation, the bushing temperature increases immediately because of the interruption of the flow of the molten glass. The filament breakage can be detected instantaneously by the abnormally high rate of temperature increase before the formation of glass beads, without using particular apparatus. When the filament breakage is not extended to the neighboring filaments, the temperature increase is too small to be detected as the filament breakage, and consequently, the unnecessary interruption of the operation can be avoided.