PURPOSE: To save a motive power of a heating device by a method wherein an operation of heating device is terminated in sequence each cylinder where a combustion has been completed therein in a system in which mixture gas in the intake pipe is heated.

CONSTITUTION: Each of the electric heaters 7 is arranged downstream of the respective carburetor 5 for each of the intake pipes 3 and these electric heaters 7 heat the mixture gas flowed in the intake pipe 3. The discharged gas sensor 8 may sense a complete combustion temperature of each of the cylinders 2 when it is cmpletely combustioned. The discharged gas temperature sensor 8 for the cylinder 2 in which a complete combustion has been made senses a complete combustion temperature, feeds the signal to the control device 9. In the control device 9, it produces an operation termination signal to the electric heater 7 to terminate an energization of the heater 7. Thereby, an excessive loading of the battery 11 may be prevented.