In a wheel train mechanism, a first speed reduction gear, a second speed reduction gear, a third speed reduction gear, a fourth speed reduction gear and an output gear are all made of a resin material. The third speed reduction gear and the fourth speed reduction gear are disposed at a rear stage of the second speed reduction gear and made of a material having rigidity higher than that of the first speed reduction gear and the second speed reduction gear. The output gear is disposed at a rear stage of the fourth speed reduction gear and is made of a material having rigidity higher than that of the fourth speed reduction gear.