To provide a combustor in which heating efficiency of radiation heat can be increased by red heating a radiation tube uniformly and strongly while suppressing generation of carbon monoxide due to incomplete combustion.

In a combustor having a radiation tube 6 suspended above a burner 4, a slit 35 is made partially in the circumference of the radiation tube 6 over the entire vertical width and a flameless hole is made opposite to the slit 35. Consequently, the slit part is not covered with flame and since secondary air flows into the radiation tube 6 through draft of flame in the vicinity of the slit 35, combustion is also accelerated the radiation tube 6 while suppressing generation of carbon monoxide due to incomplete combustion.