A transmission includes a transmission case provided with an internal gear an inner wall thereof and a multistage planetary gear mechanism housed in the transmission case. Each stage of the multistage planetary gear mechanism includes a retainer, a sun gear and a plurality of planetary gears mounted on the retainer. The plurality of the planetary gears in the last planetary gear stage of the multistage planetary gear mechanism includes four planetary gears annularly arranged between the corresponding sun gear and the inner gear in an uneven manner.