To provide a biomass fuel burner that enables an effective reduction in carbon dioxide emission, a reduction in fuel cost and stable supply of a heated medium.

A hot water boiler 1 includes the burner 2, a first heat exchanger 31 for establishing heat exchange between combustion exhaust gas from the burner 2 and water, a second heat exchanger 32 formed integrally with the burner 2, a fuel silo 4, and a screw conveyor 5 for supplying fuel in the fuel silo 4 to the burner 2. The combustion exhaust gas from fuel combustion in the burner 2 is drawn by an exhaust gas blower 81 via a return duct 62 and divided by an exhaust gas branch valve 85. One flow of combustion exhaust gas is supplied to a fuel supply pipe to preheat the fuel. The other flow of combustion exhaust gas is mixed with air and supplied to a combustion chamber of the burner 2 to be combusted again.