In a threading control system for performing a threading operation by moving, in synchronization with rotation of the main-spindle, either a cutter or a workpiece in the feeding spindle direction so that the ridge is not damaged nor the dimensional accuracy of the thread is lowered even if the main-spindle speed is variable, a threading computing section 3 and a main-spindle angle computing section 4 are provided to alter the threading start timing based on the servo-spindle acceleration time-constant of the feeding spindle.