A rotary power transmission mechanism for a photoreceptor drum device, a developing device or a fixing device used in an image forming device, comprising: a cylindrical member (22, 200); a flange member (74, 80, 82, 84) fitted to one end portion of the cylindrical member, and having a through hole passing through a center of the flange member; a shaft (52, 68) inserted through the through hole, and having an insertion hole passing through a radial direction of the shaft, the flange member being configured to transmit the rotary power of the shaft to the cylindrical member; and a pin member inserted through the insertion hole and having two end portions, which are portions of the pin member that protrude from opposite sides of the shaft, wherein the flange member has a pair of contact portions that have point symmetry with respect to an axial center of the shaft, the pair of contact portions being composed of a first contact portion (741P, 801E, 821P) and a second contact portion (742P, 802E, 822P), and when the pin member is rotated in one direction by the shaft rotating in the one direction, the first contact portion being configured to contact and to be pushed by a first end portion of the pin member and the second contact portion being configured to contact and to be pushed by a second end portion of the pin member, the first end portion being one of the two end portions and the second end portion being the other one of the two end portions, and the pin member, when rotating in the one direction, being configured to push against the first contact portion and the second contact portion, to rotate the flange member in the one direction, and thereby to transmit rotary power to the cylindrical member; characterized in that the first contact portion and the second contact portion are each a protrusion portion (741P, 742P), the protrusion portion being a protrusion of a side wall (741A, 742A) of a corresponding one of a first groove and a second groove of the flange member, and a tip portion of the protrusion portion contacts the pin member. a cylindrical member (22, 200); a flange member (74, 80, 82, 84) fitted to one end portion of the cylindrical member, and having a through hole passing through a center of the flange member; a shaft (52, 68) inserted through the through hole, and having an insertion hole passing through a radial direction of the shaft, the flange member being configured to transmit the rotary power of the shaft to the cylindrical member; and a pin member inserted through the insertion hole and having two end portions, which are portions of the pin member that protrude from opposite sides of the shaft, wherein the flange member has a pair of contact portions that have point symmetry with respect to an axial center of the shaft, the pair of contact portions being composed of a first contact portion (741P, 801E, 821P) and a second contact portion (742P, 802E, 822P), and when the pin member is rotated in one direction by the shaft rotating in the one direction, the first contact portion being configured to contact and to be pushed by a first end portion of the pin member and the second contact portion being configured to contact and to be pushed by a second end portion of the pin member, the first end portion being one of the two end portions and the second end portion being the other one of the two end portions, and the pin member, when rotating in the one direction, being configured to push against the first contact portion and the second contact portion, to rotate the flange member in the one direction, and thereby to transmit rotary power to the cylindrical member; characterized in that the first contact portion and the second contact portion are each a protrusion portion (741P, 742P), the protrusion portion being a protrusion of a side wall (741A, 742A) of a corresponding one of a first groove and a second groove of the flange member, and a tip portion of the protrusion portion contacts the pin member.