To provide a transmission device of a tractor with excellent transmission efficiency.

A transmission device of a tractor includes an engine, a hydrostatic continuously variable transmission gear that inputs a driving force output by the engine, a planetary transmission mechanism that inputs and combines the driving force output by the engine and the driving force output by the hydrostatic continuously variable transmission gear, and outputs a combined driving force, and a forward/reverse switching mechanism that inputs the combined driving force output from the planetary transmission mechanism, and outputs the input combined driving force by switching it to a forward driving force or a reverse driving force. When the hydrostatic continuously variable transmission gear is speed-controlled to a highest speed position [-Max] in a reverse rotation state, a traveling speed is set at zero, and as the hydrostatic continuously variable transmission gear is speed-controlled toward the highest speed position [+Max] in a forward rotation state, via a neutral position [N] at which the output is stopped from the highest speed position [-Max] in the reverse rotation state, the traveling speed is increased steplessly.