To obtain high intensity combustion by constituting the total area of blowing out holes arranged to a exhaust gas tank to be smaller in its ratio with the total projected area of whole burners, and setting the pressure inside a pressure feeding pipe for feeding mixed gas to the burners to be higher than the pressure inside the exhaust gas tank for blowing out the exhaust gas.

The burner port of a burner 7 is formed with multiple small holes, slits or the combination of the small holes and the slits, the high temperature combustion exhaust gas of complete combustion at the burning port part is discharged from the exhaust pipe 13 to the outside passing through exhaust holes 11 from the sack type exhaust gas tank 12 provided with many blow out holes 11 arranged at certain intervals forcing the combustion exhaust gas hit squarely the shape of the heat absorbing part 10 of the object to be heated. The total area of blowing out holes 11 arranged to the exhaust gas tank 12 per the total projection area of the burner 7 is made smaller than the total area of the burning port forming the flames of the burner 7 per the total projection area of the burner 7 and the pressure inside the force feeding pipe 5 feeding the mixed gas to the burner 7 is set higher than the pressure inside the exhaust gas tank 12 for blowing out the exhaust gas.