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 forms a continuous synthetic yarn by entangling 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 locking portion (67a, 67b, 68a, 68b) which is able to lock the continuous synthetic yarn, 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 continuous synthetic yarn when the tension becomes larger than a predetermined value; a guide portion which catches the continuous synthetic yarn to be locked to the locking portion (67a, 67b, 68a, 68b) and moves the caught portion to a predetermined inspection position so that a tension is generated in the continuous synthetic yarn; a running unit which moves a base supporting the yarn joining portion, the locking portion (67a, 67b, 68a, 68b), and the guide portion 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 determination unit which determines whether the yarn joining operation is successful on the basis of the movable amount acquired by the acquisition unit when the guide portion is moved to the inspection position. a yarn joining portion which forms a continuous synthetic yarn by entangling 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 locking portion (67a, 67b, 68a, 68b) which is able to lock the continuous synthetic yarn, 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 continuous synthetic yarn when the tension becomes larger than a predetermined value; a guide portion which catches the continuous synthetic yarn to be locked to the locking portion (67a, 67b, 68a, 68b) and moves the caught portion to a predetermined inspection position so that a tension is generated in the continuous synthetic yarn; a running unit which moves a base supporting the yarn joining portion, the locking portion (67a, 67b, 68a, 68b), and the guide portion 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 determination unit which determines whether the yarn joining operation is successful on the basis of the movable amount acquired by the acquisition unit when the guide portion is moved to the inspection position.