PURPOSE: To enable a full automatic operation and a repeated use thereof without a rupture thereof, by disposing a protective element having PTC-resistance characteristics which depend on temperature with a socket housing and a minus contact portion in series electrically, so as to make the protective element thermally connected with the socket housing.

CONSTITUTION: When an ignition plug is pressed into a socket housing 2, the wall of the ignition plug comes into contact with the wall of the socket housing 2, while a glow element of the ignition plug comes into contact with a bimetal spring contact 5. Due to such a construction, an electric current which heats the glow element flows to a minus contact portion 3 by way of a plus contact portion 4, the bimetal contact 5, the housing wall of the ignition plug, the socket housing 2, a bottom wall portion 6 and a protecting element 1. Self-heating is generated in the protective element 1 and a leakage current flows when the temperature reaches a given limit temperature. This leakage current prevents heating which exceeds an allowable range. When a cause of an overload is removed, the protecting element automatically returns to the start position.