To provide a flexible drive transmission system preventing the horizontal direction movement to the axis of a pulley pair of a belt.

This flexible drive transmission system for driving the roller of a grain mill especially is provided with the first pair of pulleys 10 and 11 rotatable on almost parallelly separated axes, around which a flexible drive belt 12 is wound. The belt is also wound around at least one idler pulley 19 supported between a turning arm pair 21 through adjusting mechanisms 23 and 24 controlling tension within the belt. An arm 21 is turnable on an insertion part 22 mounted on the arm 25 and the arm 25 is turnable around an axis 26 vertical to the axes of the pulleys 10, 11 and 19. The adjusting mechanism 27 moves the upper end edge part of the arm 25 to an inner side or an outer side and adjusts the angle position of the axis of the idler pulley 19 and thus, the track of the belt 12 is controlled and the horizontal direction movement is prevented.