PURPOSE: To absorb the fluctuation of the variation of heat-transfer in non- contact heating and to eliminate the heat-treatment variation between spindles by using a gas-combustion heater having excellent thermal response in heating and cooling and an electric heater having high temperature controlling performance and arranging the heaters in series.

CONSTITUTION: In the heat treatment in a high-speed false-twisting machine, the running yarn Y is introduced into a gas-combustion heater 1 and positioned close to but separated from a hot plate made of a porous ceramic material acting also as a burner to burn the gas and, at the same time, the temperature of the hot plate is detected by a temperature sensor 15 attached to the hot plate. The yarn is passed through a gas stream generated along the yarn-transfer path to effect the heat treatment of the yarn at a temperature controlled by a controller 16. Successively, the yarn Y is guided to an electric non-contact heater 2 and heat-treated. In the electric heater heat treatment, the temperature of the yarn delivered from the heater 2 is detected with a yarn temperature sensor 19 and the temperature of the electric heater 2 is controlled by a controller 20 to enable the heat treatment in high accuracy.