To make the cooling ability of a pair of cylinders equal in the valve system for air cooled two cylinder engines.

In the valve system of the air cooled two cylinder engine, which is provided with a timing transmitting device 60 comprising a timing drive pulley 57 driven by a crank shaft 6, a cam shaft 62 pivotally supported on a cylinder head 4, a timing driven pulley 58 belted around the cam shaft 62, and a timing belt 59 belted around the timing drive pulley 57 and the timing driven pulley 58, and wherein intake and exhaust valves 23 and 24, and 23 and 24, which correspond to a pair of cylinders 18 and 18, are connected to the cam shaft 62 continuously through intake and exhaust rocker arms 70 and 71 and 70 and 71, which are supported on a pair of rocker shafts 69 and 69 rockably, the timing transmitting device 60 is arranged between a pair of cylinders 18 and 18.