of the bath 63 with that of the cylinder 68 in more detail two baths 63 and 64 are parallelly arranged between the conveyor line belts 60 and 62 in such a way that these baths can be moved one pitch at a time by the cylinder 69 to alternately receive or discharge two or more bobbins 61 the number being determined according to the spinning speed or transfer speed of the bobbins 61 of yarn further the covers 66 and 67 alternately used for heat treating the bobbins 61 can be attached to or detached from the baths 63 and 64 by the vertical cover displacement mechanism or the cylinders 68 and 65 the loading of bobbins 61 into the baths 63 and 64 from the belt 60 and the unloading thereof from the baths 63 and 64 onto the belt 62 are implemented by a bridge conveyor 70 which can be swung to right and left to switch between the baths moreover the baths 63 and 64 are provided with belts 72 and 73 fig9 which are driven by a drive motor 71 to bring the bobbins 61 into or out of the baths 63 and 64 stoppers 75 and 76 are provided on the outlet side of these belts 72 and 73 and a stopper 77 on the outlet side of the belt 60 next a description will be entered into a series of operations of the heat treatment apparatus having the above construction at positions similar to those shown in fig9 bobbins 61 are contained in the bath 64 and under the process of heat treatment in fig9 the center line of the bath 63 is aligned with the center line of the conveyor line belt 60 so that the bath 63 can accept.