A drive force transmission mechanism includes a drive pulley, a driven pulley and an endless belt wound around the drive pulley and the driven pulley. The drive pulley and the driven pulley rotate around axes parallel to each other. At least one of the drive pulley and the driven pulley has a protruding part protruding outwardly in a radial direction around an outer circumferential face along an entire circumference. At least one axial end portion among both axial end portions of the protruding part in an axial direction of the rotational axis has a radius of curvature smaller than a radius of curvature of an axial center portion of the protruding part in the axial direction.