A power unit including an internal combustion engine with a crankshaft and a power transmission system. The power transmission system being equipped with a belt-type continuously variable transmission having a drive shaft and a driven shaft. The belt-type continuously variable transmission being disposed below the axis of the crankshaft wherein the torque from the crankshaft is transmitted to the drive shaft for rotating the crankshaft and the drive shaft rotate in opposite directions. An output shaft, linked and interlocked with the driven shaft via a gear transmission system, is disposed above the continuously variable transmission. A gear transmission mechanism includes a single idle shaft, which has an axis parallel to the crankshaft, and being arranged between the driven shaft and the output shaft for rotating the driven shaft and the output shaft in the same direction.