To provide a gear adjustable for the eccentricity of the gear itself to suppress unevenness in rotation of a driven part.

A drive transmission device is formed such that an eccentricity direction assigning ring pipe 32 is rotatably disposed at a gear body 31, and an eccentric amount regulation shaft ring 33 is slidably fitted in the square hole 37 in an oblong shape in cross section of an eccentric direction assigning ring pipe 32, and the two surfaces, making a pair, of the square hole 37 are formed in a manner to form a slope in an axial direction. The eccentric direction assigning ring pipe 32 is rotated based on a gear body 31 such that the movement direction on a radial direction of the eccentricity amount of the eccentric amount regulation shaft ring 33 is extended, as seen in a plane, to a position above a straight line to intercouple the original rotation center of the gear body 31 of a drive gear 30 and the rotation center of the eccentric direction assigning ring pipe 32. When the eccentric amount regulating shaft ring 33 is slid, the center of the bearing hole 38 coincides with the original rotation center of a gear body 31. The eccentric direction assigning ring pipe 32 is fixed at a gear body 31 by a fixed plate lock screw 35 and a direction ring fixing plate 36, and the eccentric amount regulation shaft ring 33 is fixed at the eccentric direction assigning ring pipe 32 by an eccentric shaft ring fixing screw 34.