PURPOSE: To realize sure fastening by small force by a constitution wherein an inclined surface, which is provided on a rotary gear side, a shifting member, which is shiftably provided on a stationary gear side and has an inclined surface opposedly with the inclined surface on the rotary gear side, and the shifting means of the shifting member are equipped.

CONSTITUTION: An annular inclined surface 54a, which tilts axially, is formed on the inner peripheral surface of a rotary gear 54. In an annular groove 56 surrounded by an annular notch 52c, which is opened at the inside end of the outer peripheral surface of the small diameter stepped part 52b of a stationary gear 52, and the annular inclined surface 54a of the rotary gear 54, a tapered 55, which has an annular inclined surface 55c having the same slope as that of the annular inclined surface 54a and is made into ring a having a nearly square crosssection, is fitted. By unscrewing three bolts 59 as a shifting means, the tapered ring 55, the threaded holes of which thread to the threaded parts of the bolts 59, is projected, resulting in releasing the urging between the annular inclined surfaces 54a and 55a and making the rotary gear 54 pivotable. By screwing the bolts 59 after shifting upstream side gear train including the rotary gear 54 by the distance corresponding to the length nearly ranging from the head of paper to its tail, the tapered ring 55 can be fixed to the predetermined position.