PURPOSE: To prevent the clogging by fuel while reducing the viscosity of the fuel to prevent the deposition of a wax component by providing a fuel filter with an electric resistor for reducing applied voltage to a glow plug in afterglow status.

CONSTITUTION: At the time of engine starting, a glow plug GP and a power source 1 are directly connected to each other and after complete explosion the glow plug GP and the power source 1 are connected to each other via an electric registor 13 to conduct a current to the glow plug GP. Fuel within a fuel filter 14 is heated by heat produced from the electric registor 13. After pre- heating an engine 7 while applying the voltage of the power supply 1 to the glow plug GP, the engine 7 is started, and in afterglow status after complete explosion, the voltage applied to the glow plug GP is designed to be reduced by the electric registor 13. Further, the clogging of the fuel filter 14 can be prevented because the fuel has been made to be heated.