PURPOSE: To fix a toner thermal fusin the toner material to the surface of a copying paper by providing a bass for supporting the copying paper and a slit arranged adjacently to the surface of the copying paper and condensing steam on the surface of the copying paper.

CONSTITUTION: A flash-boiler 10 receives the flow of water from a water source to a buffer cavity 24 through an inflow path 26, to evaporate the water by contact with the heated main body of the flash-boiler 10. Then, the steam is held in the buffer cavity 24, until a paper sheet S is guided and starts to move in an adjacent relation to the slit 19 of a cap 18. When the sheet S is detected, a throttle 30 is driven to obtain communication with a discharge path 28 and the pressurized steam of the buffer cavity 24 is carried to the cap 18 through the path 28. At this time, vapor pressure along the slit 19 of the flash-boiler 10 is uniformized by an othogonally crossed paths 32, to generate a knife of the stream in the opening 19. For making humidity uniform on both surfaces of the sheet S, to prevent curling, different knives of the stream are simultaneously applied to both surfaces of the sheet.