To provide a rotation driving member such as a gear, a pulley, or a roller transmitting a rotation driving force, hardly deformable even if it is formed with a synthetic resin or the like.

This rotation driving member comprises a shaft mounting portion 1 mounted to a rotary support shaft, an annular outer periphery 2 provided outside the shaft mounting portion, and a disk portion 3 for connecting the outer periphery to the shaft mounting portion. A joint portion between the shaft mounting portion and the disk portion is provided with a stiffening portion 4 that is thicker than the disk portion and has a hollow part in it. This stiffening portion 4 has a circumferentially uniform cross section shape. The stiffening portion may be disposed in a joint portion between the outer periphery and the disk portion. Additionally, the disk potion may have a shape bent in the axial line direction. In such a structure, rigidity for deformation of the outer periphery in the axial line 5 direction is increased, and deformation when a load in the axial line direction of the support shaft to the outer periphery is suppressed to be small.