A transmission device includes an output shaft, first and second input shafts, a first gear mechanism, a selective fixing device and a connecting and disconnecting device. The second input shaft is coaxially arranged on an outside periphery of the first input shaft. The first gear mechanism couples the first input shaft and the output shaft with a first gear ratio. The first gear mechanism includes an idle gear mounted on one of the first input shaft and the output shaft. The selective fixing device selectively fixes the idle gear to one of the first input shaft and the output shaft. The second gear mechanism couples the second input shaft and the output shaft with a second gear ratio. The connecting and disconnecting device selectively connects and disconnects the second input shaft to the rotating shaft.