A transmission device is provided in which a parking mechanism and a breather chamber are provided within a power-transmission case, the breather chamber being present at least above an input gear and providing communication between the interior of the power-transmission case and the exterior of the power-transmission case, wherein the breather chamber, which is present at least above the input gear and providing communication between the interior of the power-transmission case and the exterior of the power-transmission case, extends further outward than the input gear in an arrangement direction of the input gear and the output gear, and part of the parking mechanism is disposed within the breather chamber. Thus, it is possible to increase the capacity of a breather chamber while avoiding an increase in the size of a transmission device.