PURPOSE: To obtain a purging method for a furnace, for which use of a recirculating gas fan is not necessary, by making furnace purging by making use of a forced draft fan.

CONSTITUTION: When a forced draft fan 1 and an induced draft fan 8 are started, compressed air from the forced draft fan is led to a gas mixing device 4 through an air duct 6 through an intervention of a vapor type air preheater 2 and an air preheater 4. The air led to the gas mixing device 4 is divided into two one of which is entered into a furnace through a wind box 5. The other side air led to the gas mixing device 4 is sent to a sand bottom 13 of a furnace through an exhaust gas mixing duct 15, a ramification point (D) and a recirculating duct 14. The air entered into the sand bottom 13 is abosrbed by the induced draft fan 8 along with volatile gas and exhaust gas existing within a boiler B and discharged from a chimney A. With this, use of a recirculation gas fan which is likely to generate various kinds of hindrances can be avoided.