PURPOSE: To provide a device for generating vapor at high humidity.

CONSTITUTION: Water 9 in a boiler 4 is boiled by a heater 3. The vapor generated flow through the inner pipe 13 of a cooling pipe 6. Cooling oil 7 whose temperature is controlled by heating using a heater 18 in an oil bath 8, is supplied to an area between an inner pipe 13 and a sleeve 14 using a pump 17. The vapor flowing through the inner pipe 13 is cooled to a condensate by the oil, and the vapor which is not condensed is sent out passing through an exhaust pipe 26 and vapor sensor 27. Consequently the humidity of the vapor is controlled based on a dew point control method, so that the vapor of high humidity, e.g. about 100°C to 90RH%, is generated.