The arrangement of driven gears on a countershaft in a main speed change mechanism is shortened in an axial direction, whereby the main speed change mechanism can be made compact in the axial direction. Driven gears on a countershaft of a main speed change mechanism are successively arranged from front to rear in descending order of speed. A sub-low-speed drive gear for a sub-speed change mechanism is provided in front of the highest-speed driven gear on the countershaft, and the next-highest-speed driven gear serves as a sub-high-speed drive gear for the sub-speed change mechanism. A sub-low-speed driven gear and a sub-high-speed driven gear for the sub-speed change mechanism are provided on an output shaft to be engaged with the sub-low-speed drive gear and the next-highest-speed driven gear, respectively, and the highest-speed driven gear, the sub-low-speed drive gear, and the next-highest-speed driven gear are caused to overlap in a fore-and-aft direction.