PURPOSE: To provide a long service life of a radiant tube which prevents local high temp. thereof and uniformizes the temp. distribution in the length direction, by applying a simple device.

CONSTITUTION: In the radiant tube, a cylindrical heat resistant fitting material is arranged from the front part of a burner to a flame part in the inner part of the radiant tube and has inclined part whose thickness at gas exhaust side is thinned in the inner surface and length is 1.5-5 times of the distance from the front part of the burner to the peak temp. part. The max. surface temp. of this radiant tube is lowered and the surface temp. can be uniformized and the generated thermal stress is made to be small, and as the deterioration of creep fracture strength can be prevented, the service life of the radiant tube can be extended.