PURPOSE: To improve productivity by installing a pair of rotary bodies for applying a certain tension which absorbs the extension and contraction of an endless belt through the shift of a subframe and allowing the change of the distance between the driving connection parts to be absorbed when the paper size is changed.

CONSTITUTION: When the paper size is changed, the distance between an input shaft 14 and a pulley shaft 33 is changed by the advance and reverse of the right and left subframes 28 and 28A in the opposite direction with respect to the center line L. Therefore, though a timing belt 41 tends to extend or contract, a tension pulley 43 shifts together with the subframes 28 and 28A, and since a tension pulley 42 is fixed onto a main frame 3 side, the tension and slack of the timing belt 41 are offset with the tension and slack due to the contact with the tension pulleys 42 and 43. Therefore, the timing belt 41 holds a certain tension, and transmission of revolution is secured. Further, since both shafts 14 and 33 are in cantilever state, replacement of the timing belt 41 is facilitated.