To stably supply blowing air dehumidified to a humidity level same as that in winter season throughout the year to a vertical furnace with low energy consumption.

A method for blowing dehumidified air into a vertical furnace is provided which includes a step A of cooling air (x) for blowing in the furnace with an adsorption type refrigerating machine (a), and a step B of dehumidifying the air (x) through the step A with a desiccant dehumidifier system (b). In the method, fluid (y) heated with exhaust heat or/and solar heat as a heat source, is used as heating fluid for regenerating a moisture absorbent in the desiccant dehumidifier system (b) and as heating fluid for desorbing coolant vapor in the adsorption type refrigerating machine (a). The air for blowing in the furnace can be easily dehumidified with the low energy consumption even in the summer season to the humidity level same as that in the winter season.