To reduce a large amount of pumping loss.

In order to obtain satisfactory fuel consumption performance in the presently available internal combustion engine, a valve spring should be weakened. In order to obtain high performance, the valve spring should be strengthened to prevent occurrence of surging of the valve due to necessity of high speed rotation. In order to solve such conflicting problems, stepped and continuous variable valve timing mechanisms are developed. Pumping loss, however, still remains. An intake rotary shaft valve 12 and an exhaust rotary shaft valve 13 are arranged instead of the valve and valve spring. Pumping loss is thus remarkably reduced. Environment is kept clean due to due consumption performance, while an engine weight is reduced. The number of part items is also reduced. In addition, the center of gravity of the engine is lower because total height of the engine is reduced. Total width of the engine is also reduced.