To prevent the occurrence of kinky threads when thread slackening is removed.

A thread slackening removing device 1 is so designed that compressed air flow along a simplex guiding surface 5 is generated to let a thread ride on the aforesaid air flow for removing the slackness of the thread Y. By this constitution, since the flow of compressed air flows out into the three directions (downstream direction, and both the side directions) over the guiding surface 5 for widening a slackened part, the occurrence of kinky threads can thereby be prevented.