Out of a plurality of speed change gears on one of an input shaft and an output shaft, for example, on the output shaft, two adjacent speed change gears disposed via an axial space therebetween are axially fixed and rotatably fitted around the output shaft. A dog ring for a dog clutch disposed between the speed change gears is fixed around the output shaft and axially slidably fitted thereto. The speed change gears on the output shaft and the dog ring are each provided with an engagement portion for the dog clutch engaged and disengaged by axial shift of the dog ring. The two speed change gears on the input shaft meshing with the two speed change gears on the output shaft are provided integrally with the input shaft.