To reduce the axial length of an engine.

A cam shaft driving device comprises a crankshaft 27 to be drive by an engine, a transmission connected to one side of the crankshaft, an inlet cam shaft 13 and an exhaust cam shaft 14 arranged in parallel to the crankshaft, an intermediate shaft 34 arranged between each of the cam shafts and the crankshaft, and valve timing variable devices 17, 19 mounted on at least one of the inlet cam shaft, the exhaust cam shaft and the intermediate shaft at the transmission side, whereby the rotation of the crankshaft is transmitted from the other side of the crankshaft to the valve timing variable devices via the intermediate shaft.