A yarn feeding device including a preheater 23 for preheating a yarn bar Y, a first feed roller 27 for drawing the yarn bar Y from an original yarn package 11 is provided opposite to a temporary twisting and relaxation heat treating device including a second feed roller having a higher circumferential speed than that of the first feed roller 27, a temporary twisting tool 31 for twisting the yarn bar Y and a second heater 35 for carrying out a relaxation heat treatment on the temporary twisted yarn bar Y with a worker passage A interposed therebetween, a first heater 41 and a cooling plate which thermally fix a twist retroacting along the yarn bar Y are provided in portions positioned above from the yarn feeding device to the temporary twisting and relaxation heat treating device, and a winding device 51 including an automatic switching device is provided on an opposite worker passage A side of the temporary twisting and relaxation heat treating device.