PURPOSE: To detect even very small damage to a conductive film or disconnection of an electrode or of a conductive band by placing a conductive film among a plurality of electrodes installed on opposite edges of a glass pane and providing an electric power supplying circuit, which supplies the power to the conductive film via the electrode, together with detecting a temperature rise accompanying the damage to any one of these by means of a plurality of temperature sensors.

CONSTITUTION: A glass pane 1 is provided with electrodes 3a, 3b and 3c, 3d in such a manner that one end of adjacent electrodes is apart from each other. Conductive bands 4a, 4b are connected to the other ends of the electrodes 3a, 3b, respectively, while conductive bands 4c, 4d are connected to the other ends of the electrodes 3c, 3d, and a conductive film 2 is placed between 3a, 3d and 3c, 3d. In addition, temperature sensors 5b, 5c are placed between the electrodes 3c and 3d and between the electrodes 3a and 3b, respectively, all of which are on the conductive film 2, while a temperature sensor 5a is placed on a corner of the conductive film 2. When the conductive band 4b is damaged, for instance, a part of an electric current that is expected to flow from the electrode 3a to the conductive film 2 flows actually via the electrode 3a, the conductive film 2, the electrode 3b, and the conductive film 2, thereby raising the temperature in the vicinity of the temperature sensor 5c, a fact which is detected by a current detecting circuit 9.