The gear train comprises: an input gear having Zi number of teeth; an output gear having Zo number of teeth; and an idle gear between the input gear and the output gear, transmitting power from the input shaft to the output shaft at a pre-set gear ratio. The idle gear is provided with: a first gear meshing with the input gear and having Zdi number of teeth; and a second gear meshing with the output gear and having Zdo number of teeth. The number (Zi) of teeth on the input gear and the number (Zo) of teeth on the output gear have at least one first common factor (a), and the number (Zdi) of teeth on the first gear and the number (Zdo) of teeth on the second gear have at least one second common factor (b), and the first common factor (a) and the second common factor (b) are relatively prime.
