A yarn joining system for synthetic yarn used in a textile machine including a creel stand supporting a yarn supply package obtained by winding a synthetic yarn made of synthetic fibers on a bobbin by a yarn supply package support portion, a processing device processing the synthetic yarn supplied from the yarn supply package, and a winding device forming a winding package by winding the synthetic yarn subjected to the processing and used to draw a yarn end from each of a pair of two yarn supply packages supported by the yarn supply package support portion and to join the yarn ends, the yarn joining system for synthetic yarn comprising a yarn joining portion which entangles a first yarn end corresponding to a yarn end of the one yarn supply package and a second yarn end corresponding to a yarn end of the other yarn supply package; characterized in that the yarn joining system for synthetic yarn further comprises: a guide portion which guides the first yarn end and the second yarn end to the yarn joining portion; a locking portion (67a, 67b, 68a, 68b) which locks the synthetic yarn when the guide portion guides the first yarn end and the second yarn end to the yarn joining portion, the locking portion (67a, 67b, 68a, 68b) being provided so that a movable amount from an initial state increases in response to a tension acting on the synthetic yarn when the tension becomes larger than a predetermined value and is urged so as to return to the initial state when the tension acting on the synthetic yarn is equal to or smaller than the predetermined value; an operation mechanism (62,63) which drives the yarn supply package support portion supporting the yarn supply package of the creel stand so as to be rotatable in a feeding direction; a running unit which moves a base supporting the yarn joining portion, the guide portion, the locking portion (67a, 67b, 68a, 68b), and the operation mechanism (62,63) along the creel stand; an acquisition unit which acquires a movable amount of the locking portion (67a, 67b, 68a, 68b) with respect to the base; and a drive control unit which controls a feeding amount of the synthetic yarn fed from the yarn supply package by controlling the operation mechanism (62,63) so that the movable amount is in a first predetermined range. a yarn joining portion which entangles a first yarn end corresponding to a yarn end of the one yarn supply package and a second yarn end corresponding to a yarn end of the other yarn supply package; characterized in that the yarn joining system for synthetic yarn further comprises: a guide portion which guides the first yarn end and the second yarn end to the yarn joining portion; a locking portion (67a, 67b, 68a, 68b) which locks the synthetic yarn when the guide portion guides the first yarn end and the second yarn end to the yarn joining portion, the locking portion (67a, 67b, 68a, 68b) being provided so that a movable amount from an initial state increases in response to a tension acting on the synthetic yarn when the tension becomes larger than a predetermined value and is urged so as to return to the initial state when the tension acting on the synthetic yarn is equal to or smaller than the predetermined value; an operation mechanism (62,63) which drives the yarn supply package support portion supporting the yarn supply package of the creel stand so as to be rotatable in a feeding direction; a running unit which moves a base supporting the yarn joining portion, the guide portion, the locking portion (67a, 67b, 68a, 68b), and the operation mechanism (62,63) along the creel stand; an acquisition unit which acquires a movable amount of the locking portion (67a, 67b, 68a, 68b) with respect to the base; and a drive control unit which controls a feeding amount of the synthetic yarn fed from the yarn supply package by controlling the operation mechanism (62,63) so that the movable amount is in a first predetermined range.