A gear transmission permits an increased flexibility in the layout of shift gears and reduces the axial size of the transmission by providing an engaging portion on a specific one of the shift gears. The gear transmission includes a main shaft having drive gears, a countershaft having driven gears adapted to be respectively driven by the drive gears to set predetermined shift positions, and shifters engageable with specific gears of the drive gears and the driven gears. The specific gears respectively have engaging portions for engaging the shifters, whereby the main shaft rotationally drives the countershaft through any one of the shifters and any one of the specific gears at any one of the predetermined shift positions. Of the engaging portions, the outer diameter of the engaging portion is larger than the outer diameter of a toothed portion of the drive gear as one of the specific gears.