An internal combustion engine includes a crankshaft, a mainshaft, a clutch and a countershaft. The mainshaft is for a transmission and holds a transmission gear train. The clutch is disposed on the extension of the mainshaft. Furthermore, the countershaft for the transmission holds a gear train to mesh with the transmission gear train. Accordingly, power is transmitted in order from the crankshaft to the clutch, the mainshaft, and the countershaft. This arrangement ensures better knee-grip when the internal combustion engine is mounted on a vehicle and improves in-vehicle mountability by reducing the width and the axial length of the internal combustion engine. A mainshaft is disposed about immediately below the crankshaft. At least a part of the mainshaft is provided at the same axial position as the crankshaft.