Five receiving holes 2e extending along the axis of a circular cylindrical portion 2b of a housing are formed in the circular cylindrical portion 2b. Those five receiving holes 2e are arranged at equal intervals in the peripheral direction of the circular cylindrical portion 2b. Each receiving hole 2e rotatably receives therein a planetary gear 5. The respective planetary gears 5 are in engagement with an inner gear 3 and a sun gear 4. The teeth of the inner gear 3, the sun gear 4 and the planetary gears 5 are helical teeth. The numbers of teeth of the inner gear 3, the sun gear 4 and the planetary gears 5 are set to 36, 24 and 6, respectively. As a result, the numbers of teeth of the inner gear 3 and the sun gear 4 located between adjacent two planetary gears 5 in the peripheral direction become 7.2 and 4.8, respectively, which are values each having a fraction below the decimal point.