A cam structure includes a camshaft and a cam lobe. The cam lobe includes a base cam and a roller which is provided at a distal end portion of a valve lifting portion. A first plus peak of acceleration of a valve is caused on starting a valve lift, a second plus peak is caused on finishing the valve lift, and the acceleration of the valve becomes minus between the first peak and the second peak. An outside diameter and mounting position of the roller are set so that a follower portion of a valve mechanism rides on to the roller from the valve lifting portion at a side of the starting of the valve lift and rides on to the valve lifting portion from the roller at a side of the finishing of the valve lift between the first and second peaks.