A valve operating mechanism of an internal combustion engine, comprises a drive cam rotatable with and eccentric to a drive shaft; a link arm having a circular opening in which the drive cam is rotatably received, so that rotation of drive cam about an axis of the drive shaft produces a swing movement of the link arm; a pair of swing cams swingably disposed on the drive shaft at both sides of the drive cam, the swing cams being connected through a movement transmission mechanism to the link arm to be swung when the link arm is subjected to the swing movement; a pair of swing arms respectively actuated by the swing cams for carrying out an open/close operation of a pair of engine valves; a pair of spring retainers respectively provided by the pair of engine valves; and a pair of valve springs respectively held by the spring retainers and biasing the engine valves in a close direction. A lubricating oil passage is formed in the drive cam. The oil passage has one end exposed to an oil feeding passage formed in the drive shaft and the other end exposed to a minute clearance defined between a cylindrical outer surface of the drive cam and a cylindrical inner surface of the circular opening of the link arm.