PURPOSE: To contrive elimination of waiting time for preheating of an evaporation chamber, by a method wherein combustion is carried out with gaseous fuel at the begining of the combustion and after that a combustion method is switched to the one with liquid fuel.

CONSTITUTION: At the time of starting of combustion, compressed gaseous fuel in a gaseous fuel tank 110 whose gas flow is adjusted to a fixed one through a nozzle 35 is supplied to an evaporation chamber 19. Air for combustion supplied into the evaporation chamber 19 through a rotation of a combustion fan 8 is mixed with the gaseous fuel, which is spouted out from a burner head 26 and ignited by an ignition electrode 32 for premixture combustion. When a temperature of an evaporation cylinder reaches to a fixed temperature, simultaneously with stoppage of supply of the gaseous fuel by closing a solenoid valve 113 liquid fuel is supplied into the evaporation chamber 19 from a liquid fuel supply pipe 24 for entering into a liquid combustion stage. With this, waiting time for preheating of the evaporation chamber becomes unnecessary and an instantaneous combustion becomes practicable when it is required to use.