To protect a ceramic tube against cracking due to an inner and outer temperature difference by inserting the inlet side of a tube into a tunnel provided at the lower part of a burner fixing wall and providing the tunnel with an exhaust pipe in the vicinity of a tube inlet thereby discharging a part of a combustion exhaust gas of a burner from an exhaust pipe.

When a steel material 1 is carried in a ceramic tube 2 and heated by means of an alternating combustion burner 3 from the outside of the tube 2, a tunnel 5 having an open outlet side is provided at the lower part of a burner fixing wall 4 and inserted with the inlet side of the tube 2. The tunnel 5 is provided with an exhaust pipe 6 in the vicinity of the inlet of the tube 2 so that a part of a combustion exhaust gas of the burner 3 is discharged from the exhaust pipe 6. According to the arrangement, the ceramic tube 2 can be protected against cracking due to an inner and outer temperature difference in the vicinity of the inlet.