A speed reducer includes a planetary gear, a first ring gear and a second ring gear. The planetary gear is a helical gear and has planetary gear teeth each of which is tilted relative to an axis of the planetary gear. The first ring gear is a spur gear and has first ring gear teeth which extend in parallel with an axis of the first ring gear and are configured to mesh with the planetary gear teeth. The second ring gear is a spur gear and has second ring gear teeth which extend in parallel with an axis of the second ring gear and are configured to mesh with the planetary gear teeth.