PURPOSE: To optimize spinning geometry via a fuzzy control mechanism and to produce high quality spun yarn in high productivity.

CONSTITUTION: In the ring frame, the spinning force or thread tension and/or the frequency of end breakage are detected via a sensor 25 for detecting the thread tension and a finger 27 which are interposed between a pair of feed rollers 13 and thread guide 15 on the outlet of a draft mechanism 5, and the detected signals 29 are inputted to a control unit 31 equipped with a fuzzy mechanism. Otherwise, the measured values such as temperature and/or humidity in air, are inputted as parameters of the boundary condition of the ring frame in the control unit 31. The control unit 31 commands a control mechanism 39 which is driven by an actuator 33 and a spindle in accordance with the input signals 29 and controls the vertical positions of the thread guide 15 and an anti-node ring 33, the horizontal interval between the draft mechanism 5 and the spindle 11 or the revolution number of the spindle so that the spinning force, the thread tension and the frequency of the end breakage do not exceed the predetermined values or are minimized. Thus, the spinning geometry is optimized.