PURPOSE: To enable to detect current with no need of connecting a current detecting circuit, by using glow plugs having a prescribed temperature coefficient of resistance, and detecting the magnetic field produced by the current passed through the glow plugs.

CONSTITUTION: A current detecting and amplifying block 30 is disposed at a position of a conductor 5 located on the downstream side of a main relay 1 and it is connected to a glow plug controlling circuit 32 by a bundle of conductors 31 for feeding current and transmitting the output signal of the block 30. The current detecting block consists of a magnetic circuit 8 which is constituted by a material like iron or ferrite having a high magnetic permeability that is disposed in the manner of surrounding the conductor 5, and a detector element 6 consisting of a magnetic resistance element serving as a Hall-effect element is inserted to a portion of the circuit 8. By thus detecting and controlling the temperature of glow plugs by a magnetic means, it is enabled to enhance reliability of the circuitry.