Abstract: A thread-tension regulating device for multi-thread sewing machines comprises a speed change mechanism and a thread-tension regulating mechanism. The speed change mechanism has an infinite speed variator and pulleys for operating the variator. The thread-tension regulating mechanism comprises a cam shaft rotated by the speed change mechanism, cams fixed onto the cam shaft, operating rods reciprocated by the cams, pairs of tension discs mounted on the corresponding operating rods, and biasing elements for periodically imparting tension to threads which pass between the paired tension discs according to the reciprocating movement of the operating rods. By selecting the shape, dimension and angular phase differences of the cams, a biasing force between the paired tension discs, and the rotational speed of the cam shaft as given by the speed change mechanism, a variety of patterns can be formed on cloth by the multi-thread sewing machine.