To provide a transmission device for a tractor in which PTO inverse rotation specifications or super-high speed running specifications can be provided easily and at low cost.

A clutch housing 2 including a main clutch 1 and a transmission case 5 including a main transmission device 3 and an auxiliary transmission device 4 are connected to each other by a transmission cover 8 to cover a running transmission shaft 6 and a PTO transmission shaft 7 for transmission between each other, and an inverse rotation clutch gear 10 changeable to an inverse rotation position by an inverse rotation clutch 9 on the running transmission shaft 6 and a gear case 12 to contain a counter gear 11 on the PTO transmission shaft 7 can be installed on the back side of the clutch housing 2 inside the transmission cover 8. Instead of the gear case 12, a gear case to contain a transmission clutch gear changeable to a transmission position by a transmission clutch on the running transmission shaft 6 and a counter gear on the PTO transmission shaft 7 can be installed with the inverse rotation clutch 9 and the inverse rotation clutch gear 10.