A driving force transmission device includes a rim having a toothed outer periphery, with bottom lands being formed between respectively adjacent teeth and top lands being formed at teeth crests. Outer ribs are disposed at positions on an inner peripheral surface of the rim corresponding to the bottom lands, whereby there is neither variance in angle speed nor uneven rotation of a member driven by the transmission device. Moreover, the outer ribs are radially offset from inner ribs, whereby tensile forces generated between the outer and inner ribs and caused by material shrinkage cancel each other out and defective molding of teeth crests can be minimized. Accordingly, the radius of an imaginary circle formed by joining the top lands is uniform in a circumferential direction.