PURPOSE: To make the temperature of a heater uniform by concentrically by providing a group of fuel pipes at a position close to the inner face of a combustion drum.

CONSTITUTION: The combustion air sent by an external air fan is preheated at an air preheating element 3, passes over a combustion device liner 4, and is sent into an air buffer 9 after passing through between fuel pipes 7 by a flow guide plate 12 as shown by arrows 'a'. The air sent to the air buffer 9 flows outside the group of fuel pipes through between the fuel pipes 7. On the other hand, fuel is sent from a fuel pipe into a fuel buffer 10 through a connecting port 11, distributed to the fuel pipes 7, spouted from a lot of fuel spouting holes 8 provided on the fuel pipes 7, mixed with the combustion air, spouted from burner ports 6 and burnt. (the fuel flow is shown by arrows 'f') The combustion flames 14 horizontally, radially stretch and strike perpendicularly against a heater 1. Since the burner ports 6 are uniformly arranged in the longitudinal and circumferential directions and the flames strike across the whole length of the heater 1, the temperature of the heater 1 can be kept uniform.