To provide a boiler capable of combusting solid biomass fuel such as firewood, woody fuel, or carbide by a batch system over a long period of time, easily performing control of a combustion amount, dormancy and recombustion, and obtaining clean exhaust gas, and a hot air generating device with high thermal efficiency using the boiler.

Clean exhaust gas is obtained by means comprising: injecting dried solid biomass fuel by the batch system into a furnace having high heat insulation properties; introducing highly concentrated flue gas generated in primary combustion to a vertical type smoke flue reaction column duct; performing secondary combustion for combusting light weight gas with preheated air; performing primary combustion for combusting flame-resistant gases and ash dust by a burner; and performing quaternary combustion for completely combusting remaining incompletely combusted portions by a honeycomb type catalyst. The combustion amount and the hot air temperature of the furnace are controlled by interlocking carbon monoxide and ash dust in the exhaust gas with combustion conditions of the primary combustion to the quaternary combustion.