PURPOSE: To enable to heat at a temperature below 100°C by a method wherein a suction pump is attached at the rear end of a heating pipe, at the head-on of which a regulating valve is attached, in order to forcibly suck and circulate steam and at the same time so as to expand the steam adiabatically after throttling by the regulating valve in order to lower the temperature of the steam.

CONSTITUTION: The steam fed from a steam boiler 2 is sucked by a suction pump 7 so as to forcibly circulate through a heating pipe 4. At this time, the steam is properly throttled by a regulating valve 3 and expanded adiabatically, resulting in decreasing its pressure below the atmospheric pressure. The resultant steam is regulated its temperature below 100°C and used for the heating of material, which is easily denaturized thermally. In addition, the thermal efficiency is improved, becaused the steam is forcibly circulated.