PURPOSE: To distribute hot air to the respective heating cylinders in air streams of equal flow rate, by means of fixed restricted parts such as orifices of the same diameter provided at the hot air blowing inlet corresponding to the respective heating cylinders.

CONSTITUTION: An apparatus for heat-treating yarns capable of passing hot air distributed in the respective hot air blowing inlets 12 through orifices 15-1, 15-2, 15-3 and 15-4, restricting the holes of the orifices to small holes to increase the ratio of differential pressure before and after the orifices to the pressure loss of the whole hot air paths from the hot air supply source to the heating cylinders 10, and distributing the hot air equally to the respective hot air blowing inlets 12 through the respective orifices set at the same restricted diameter at the same position.