Abstract: A reduction gear can provide a relatively large reduction gear ratio by changing the number of gear teeth but without increasing the number of gears, thereby simplifying the structure thereof. The power transmission flow in the reduction gear occurs as follows. In one embodiment, power simultaneously flows to both a first sun gear and to a second sun gear. From the first sun gear, power flows through, in succession, a planetary gear, a ring gear, a carrier and finally to an output ring gear and output shaft. From the second sun gear, power successively flows through a first differential gear and a second differential gear to the output ring gear and the output shaft. In another embodiment, power flows, in succession, through an input sun gear, a first differential gear and a second differential gear, to simultaneously both a differential sun gear and to a differential ring gear.