PURPOSE: To prevent the surplus or shortage of heat quantity for heating by a method in which a heater is connected to power source through a controller to be operated by a revolving number detector, and fuel is heated proportionally to the revolving number of engine.

CONSTITUTION: When a power source switch 18 is turned on and a filter is blinded with wax component due to the lowering of fuel temperature during the operation, a detection switch 15 is turned on. When revolving number is higher than an electrification-cut revolving number Nc in this case, since non-inversion input voltage of a comparator 13 from an F/V converter 74 is higher than the inversion input voltage, a transistor 14 is electrically energized. Since a coil 91 is energized and a switch 92 is turned on, a heater coil 6a is electrically energized, and current is flowed proportionally to the revolving number of engine. When the revolving number is reduced beyond the value Nc, since the non- inversion input of the comparator 13 becomes lower than the inversion input voltage, electrification to the transistor 14 is stopped. Electrification to the coil 91 is thus stopped, the switch 92 is opened, and supply of current to the heater 6a is also stopped. The wasteful consumption of battery 10 and the overheating of heater 6 can be prevented.