A variable valve characteristic mechanism has a valve operation cam pivotally supported on a camshaft and a driving mechanism rocking an interlocking mechanism around the camshaft, the interlocking mechanism rocking the valve operation cam by a driving cam integrated into the camshaft. The engine valve starts to open and close in a damping portion of the valve operation cam, and the driving mechanism rocks the valve operation cam via the interlocking mechanism, thereby controlling an opening timing and a closing timing of the engine valve. A cam swelled portion of the driving cam has a constant velocity portion in which a lift velocity is constant, and the constant velocity portion is provided over an angular width where the opening timing in a most advanced angle position of the opening timing of the engine valve and the opening timing in a most retarded angle position of the opening timing are included.