PURPOSE: To heat uniformly the work with ease at a low cost by segmenting the inside of a heat treatment furnace to plural zone units, and controlling the heating in each zone according to the position of the upset part and pipe body part of the work.

CONSTITUTION: The fuel system Fu and air system Ai in the upset part 2-1 of the work 2 are shown by a bold solid line and a broken line, and the pipe body part is shown by a fine line. If, for example, the part 2-1 is in a zone B and the pipe body part in a zone F, shutoff valves 17, 18 in front of burners in the zones A, B are opened and 17', 18' are closed. The temps. in the zones A, B are detected with detecting ends (a), (b), and are inputted to controllers 13, 13'. The flow rates of fuel and air are detected with flowmeters 11, 11' and are inputted to controllers 13, 13'. The controllers 13, 13' judge input information and determine the opening of control valves 15, 15' to optimum setting. Shutoff valves 17', 18' are opened and shutoff valves 17, 18 are closed in the heating zone F for the pipe body part, then the openings of valves 16, 16' are determined by the similar operation.