A venturi nozzle (1) that is disposed upstream of a fan (20) and that mixes air for combustion and fuel gas by way of the intake pressure of the fan (20) comprises: a nozzle portion (12) by which airfor combustion is introduced and that is formed so that the diameter thereof decreases toward the downstream side; a mixing portion (13) by which air for combustion and fuel gas are mixed, that is formed so that the diameter thereof increases toward the downstream side, and that is disposed on the downstream side of the nozzle portion (12); and a fuel gas introducing portion (15) by which fuel gasis introduced and that is disposed between the nozzle portion (12) and the mixing portion (13). A plurality of ridges (16) that extend in the circumferential direction and that are disposed at a prescribed interval in the flow direction of the air for combustion are formed on the inner face of the nozzle portion (12).