PURPOSE: To remove shifting of a furnace core tube to prevent a decrease in temperature of the furnace core tube so that its safety can be improved by preventing an explosion accident of the furnace core tube due to low temperature, by providing a diffusion furnace device with mechanism for fixing the furnace core tube.

CONSTITUTION: Four screw rods 11... are installed on a plate 4 of a diffusion furnace, and a furnace core tube press 1 having recessed parts 1a is screwed with the screw rods 11, facing the furnace core tube 2, and an end of the furnace core tube 2 is fitted/ held against the recessed parts 1a of the furnace core tube press 1 so that the furnace core tube 2 is suppressed to be shifted in the axial direction inside an equalizing heat pipe 3. Besides, a nozzle 9 is mounted passing through the press 1. Thus, even if contact friction force is caused between a mother board 6 and the furnace core tube 2 when the mother board 6 is carried into the furnace core tube 2, the furnace core tube 2 is suppressed to be shifted in the axial direction because the end of the furnace core tube 2 is fixed by the press 1. Therefore, because a burning part 10 in the furnace core tube 2 is always located within a heating range of a heater part 5, an explosion accident of the furnace core tube 2, to be caused by the burning part 10's being at low temperature, can be prevented.