PURPOSE: To provide the titled system when devices for a series of treatments to spun yarn such as steam jet, drain connection and vacuum suction are equipped during the conveyance of the spun yarn from a fine spinning machine, etc., thus preventing adverse effects caused by release of heat-setting vapor in the fine spinning chamber.

CONSTITUTION: A spun yarn 50 (wound on a bobbin) which is delivered from a fine spinning machine or winding machine and being conveyed on a conveyor 51 is fed to a heat-setting chamber 1 followed by closing a door 2. While the chamber 1 is moving round, a connecting fitment 22 is linked to the chamber 1 at C-position to perform a vacuum suction of the inner space of the chamber 1. At the C-position, a vapor valve 26 is opened to supply a vapor from an electric vapor generator 23, temperature adjustments being made at EWN- positions; the spun yarn 50 is thus heat-treated and the twist fixed. When reached O-position, vapor supply is stopped; and when reached P-position, a connecting fitment 31 is connected to perform a drain followed by vacuum suction; the spun yarn 50 is then returned to the conveyor 51 at Q-position.