To stop power supply to a coil intermediately during burning of the coil by arranging a thermal means consisting of thermally soluble member melting at a temperature more than a given value and a conductor arranged on the both sides through it on the surface of the coil so that it crosses the winding direction of the coil.

A short wire 5 adhered on the surface of a secondary coil 23 is equipped with a first wire 51, an insulation film 53 covering the first wire 51, and a second wire 52 wound around the insulation film 53, and the insulation film 53 is made of nylon or others melting at a given temperature. Because the short wire 5 is arranged so that it crosses the winding direction of the secondary coil 23, even if burning occurs at any part of the secondary coil 23 the short wire always has a part bringing into contact with a burning part, where the insulation film 53 intermediately melts and the first wire 51 conducts to the second wire 52. This conduction causes current flowing through the second wire 52 to be entered into a microcomputer 62 of a detecting circuit 6. The microcomputer 62 controls a switching circuit 8 to be released and breaks power supply to a primary coil 22.