PURPOSE: To heat uniformly a blank material and to prevent formation of scale by using an annular groove formed around a hole for taking out the blank material as a circulation passage for combustion flame and heating the bar-like material.

CONSTITUTION: A gaseous mixture ejected from a petroleum burner installed to a burner installation cylinder 14 flows along the annular groove 11 of a hearth 2. When the gaseous mixture is ignited, the combustion flame thereof circulates along the groove 11 and the combustion gas thereof is discharged from a smoke release cylinder 15. The annular groove 11 functions as a circulation passage for the combustion flame. The blank material for forging in respective refractory tubes 13, 13W installed in the groove 11 is heated via the tubes 13, 13W. Since the heat of combustion is uniformly acted around the blank material via the tubes by the abovementioned method, the uneven heating of the blank material is obviated, and the blank material is enclosed of the refractory tubes around the same and is interrupted of the contact with the air and the gaseous mixture in the stage of heating, whereby the formation of scale is obviated.