PURPOSE: To prevent the contamination of impurities in a vent hole provided in a furnace body and improve the thermal efficiency, by placing a repair rod which can be cooled and is mounted on the tip of a holder movable in the vertical, transverse and axial directions on a frame.

CONSTITUTION: A holder 4 capable of being advanced and retreated by an operation cylinder 2 provided on the back end side of a frame 1 installed facing a vent port 15 of a furnace body 10 of fused silica production furnace and lifted and lowered by a roll 5 having a lifting cylinder 7 provided at the front end part of the frame 1 is placed on the frame 1. A repair rod 3 in which the interior is divided into a water supply part (4a) and water discharge part (4b) and the respective parts (4a) and (4b) are connected to a water supply pipe and water discharge pipe through connection ports 9 and 9 is detachably mounted on a protruding water supply port 19 at the tip of the holder 4.