PURPOSE: To provide a light weight burner unit of less expensive price, promote cooling operation and to enable the prevention of thermal strain by a method wherein a burner main body is formed out of a thin walled steel plate and a cooling air passage is arranged at an outer circumference of the burner main body.

CONSTITUTION: An infrared ray burner 1 is provided with a horizontally elongated combustion chamber 2, a combustion plate 3 made of porous ceramic material is arranged at the front surface of a burner main body 1a made by a rectangular steel pipe 25 having a rectangular cross section is provided at an outer circumference. Cooling air sent from air-supplying port 36 to the air passage 26 flows within the air passage along the outer circumference of the burner main body 1a and then flows out to the outer part through through-holes 37 the through-holes 38 in sequence. While the air passes through the air passage 26, the side surfaces of the burner main body 1 are cooled. The air flowing out of the through-holes 37 will cool a guide plate 27 and the air flowing out of the through-holes 38 will cool the rear surface of the burner main body 1a and the side surfaces of the mixing gas chamber 5. The burner is formed out of a thin-walled steel plate, thereby making a light weight burner; thus, a cooling after diminishing of flame is rapidly performed.