PURPOSE: To prevent the breakage of wire by securing the proper electric conduction until the temperature distribution of a glow plug is stably uniformalized, by continuously varying the standard voltage corresponding to the glow plug temperature in accordance with the variation of the heat generation distribution state of the glow plug.

CONSTITUTION: When a key switch 2 is turned-ON, a transistor Tr4 is turned-ON by the operation of a driving circuit 3, and the voltage of a battery 1 is applied onto glow plugs 6aW6d, and the glow plugs are speedily heated. Immediately after the key switch 2 is put into ON-state, a condenser 38 is charged through a resistance 39, and the terminal voltage increases gradually, and a standard voltage V34 increases to the standard voltage corresponding to about 900°C from the low voltage value corresponding to about 600°C of the glow plug temperature. When the detection voltage corresponding to the glow plug temperature variation detected by a detection resistance 7 reaches the above-described standard voltage, the conduction current applied to the glow plugs 6aW6d is limited.