PURPOSE: To automatically and smoothly perform the charging of a steel piece into a furnace and the extraction thereof to the outside of the furnace, by a method wherein the position of the steel piece in a walking beam type heating furnace is detected by a γ-ray detector and the stroke of a movable floor is regulated by a detection signal.

CONSTITUTION: In heating a steel material 1 in such a state that the steel material is moved while supported by the movable floor 12 and the fixed floor 11 in a walking beam type heating furnace, the steel material 1 on the movable floor 2 is detected by a γ-ray steel material detector consisting of a γ-ray source and a γ-ray detector 17 and the advance stroke of the movable floor 12 is detected by a pulse counter 19. The position of the steel material 1 is calculated on the basis of the signals from the γ-ray steel material detector 15 and the pulse counter 19 by an operator 20 while the desired stroke of the movable floor 12 is calculated on the basis of the signals from the pulse counter 19 and the operator 20 by a stroke control apparatus 21 and the charging and the extraction of a steel piece 1 are smoothly performed by adjusting the stroke of the movable floor 12 through a hydraulic control apparatus.