A valve driving device of an engine under the present invention is provided with a cam shaft (71) for exclusive use of intake for driving intake valves and a cam shaft (72) for exclusive use of exhaust for driving exhaust valves. At least either the intake valve or the exhaust valve is pluralized and a transmitting (133) member is trained between the cam shaft with more valves to drive and an engine output shaft. Gears (143) to mesh with each other are provided on said two cam shafts (71,72). Under the above composition, only one pulley (132) on the driven side or only one sprocket is required and therefore compactness of the engine can be realized. Also, rotation of the cam shaft having more valves to drive and large torque fluctuation can be stabilized and reliability of gears can be improved, without reinforcing the gears.
