Provided is a threading device for an overlock sewing machine for which the overall device has been made compact and with which the operator can securely perform threading with one hand. The device comprises: a threading shaft, which moves vertically and is supported to be rotatable with the shaft center as the central axis; a threading hook having a hooking section, which is fastened to the lower end of the threading shaft and is capable of being inserted into the eye of a sewing needle, and a first guiding section, which guides the hooking section to the eye of the needle; and a thread-guiding member, which guides the upper thread to the hooking section near the limit of descent of the threading shaft. The device rotates the threading hook by a prescribed angle so that the hooking section is inserted in the eye of a sewing needle near the limit of threading shaft descent. The threading hook and the thread-guiding member are rotated in mutually opposite directions and the thread-guiding member is rotated by a prescribed angle so that a thread-catching section passes the position of the sewing needle and intersects with the sewing needle near the limit of threading shaft descent. The threading shaft is moved according to the relative positional relationships between the eyes of multiple sewing needles.