To provide a system capable of joining an elastomer yarn to a sliver fed through a drafting zone of a spinning frame, forming an elastomer core/wrap yarn and enabling high-speed mass production at a low cost.

This system for forming an elastomer core/wrap yarn is obtained by installing a threading means 240 for feeding an elastomer yarn 44 to drafting zone 42 of a spinning frame, a feeding means for controllably feeding the elastomer yarn 44 to the threading means 240 and an elastomer yarn sensing means for sensing the elastomer yarn 44 passing through the threading means 240 and is capable of joining the elastomer yarn 44 fed through the drafting zone 42 to a sliver S and forming the elastomer core/wrap yarn. Furthermore, a means capable of controlling the interaction of the threading means 240 with the feeding means and the elastomer yarn sensing means is preferably installed therein.