PURPOSE: To provide the titled such system that a winding small frame revolution stopper is actuated in response to the displacement of a slit gauge caused by change in the frictional resistance of a traveling yarn when it passes the slit of said gauge, thus entirely preventing the occurrence of fineness jump.

CONSTITUTION: The objective system can be provided as follows: a slit gap of a slit gauge 10 is set so that when a yarn Y of large fineness beyond a specified limit passes, it is subjected to high frictional resistance. When the slit 10 is pushed upward, a fineness jump-detecting piece 13 is moved round the shaft 19 to be latched onto a hook 22 to effect moving a brake 37 in the direction of the arrow through a fineness jump-detecting lever 18, wire 24 and sensing lever 26 to stop a winding small frame 8 and push up a hourglass-shaped wheel 6, thus resulting in loosening said yarn Y.