To provide a variable valve opening characteristic internal combustion engine improved in the degree of freedom in setting valve opening characteristics while employing comparatively simple structure.

A roller 28 is brought into rolling contact with the cam lobe 4a of an exhaust camshaft 4. A roller shaft 27 is brought into rolling contact with a circular arc slipper surface 6a formed on an exhaust rocker arm 6. The rotational center of the exhaust camshaft 4 is positioned in a direction facing the slipper surface 6a. Within the moving range of a control shaft 25, with the roller 28 brought into rolling contact with the base circle Cb of the cam lobe 4a, the circular arc center Ps of the slipper surface 6a is offset obliquely upward in drawing with respect to the center Pc of the base circle Cb. Therefore, the radius of curvature R of the slipper surface 6a is set significantly large in comparison with the case where the center Pc of the base circle Cb conforms with the circular arc center Ps of the slipper surface 6a.