PURPOSE: To keep almost constant the temperature of a vaporization section and shorten the time required for preheating it by setting the heat quantity received by the vaporization section a little larger than specified and at the same time detecting its temperature and maintaining almost constant the vaporization temperature by adjusting the degree of cooling at the vaporization section and at the same time keeping the degree of cooling of the vaporization section at minimum during the term of preheating the vaporization section.

CONSTITUTION: After ignition a control section 26 rotates a damper 24 based on the output from a temperature detection section 25 and adjusts the volume of secondary air that is made to flow to the side of cooling 19. Namely with this arrangement it is possible to keep almost constant the temperature at the vaporization section without relying on the electric supply to a heater 14. Further, since it is possible to maintain the temperature of the vaporization section 11 always near a target temperature, the durability of the device is improved and it is possible to shorten the time of preheating by the heater.