To prevent the erosion of an electrode liner in accompany with the spark caused by flue dust attached and accumulated between an electrode of an electric-arc furnace and the electrode liner.

Thermometers 9, 10 are mounted on water cooling passages 7, 8 on an inlet side and an outlet side of a water cooling jacket 6 mounted on the electrode liner 5 of the electric-arc furnace, signals of the thermometers 9, 10 are taken in an operation warning device 11, and the operation warning device 11 compares temperature signals to determine the deviation, compares the deviation with a set value, and issues the warning when the deviation is over the set value. When the flue dust E attached to the electrode 3 is attached and accumulated on the electrode liner 5, the electric resistance between the electrode line 5 and the electrode 3 is increased, as a result, the spark is generated, and the electrode liner 5 generates the heat, whereby the temperature of the thermometer 10 is raised and the warning is issued. Here, the flue dust E attached and accumulated on the electrode line 5 is removed by a sander and the like. As the generation of spark is prevented, the cost for repairing the electrode liner, and the number of times of replacing the electrode liner and removing flue dust can be reduced.