PURPOSE: To improve the combustion efficiency thus to improve the fuel consumption and to prevent detonation by setting the crank lift characteristic such that the volume of combustion chamber will vary with the crank angle while executing the advance/delay angle control of ignition timing.

CONSTITUTION: Since an intake valve 8 and an exhaust valve 9 are slided while interlocking with the rotation of crankshaft, the volume of combustion chamber 15 will vary with the crank angle thus to vary the compression ratio with the crank angle. Consequently, advance/delay angle control of ignition timing of ignition plug is executed through utilization of the crank lift characteristic thus to vary the compression ratio. When leading such that the ignition timing is brought near to the top dead center under partial load, high compression ratio is achieved to improve the combustion efficiency thus to improve the fuel consumption. While under full-open load, the ignition timing is advanced to lower the compression pressure when forming the nucleus of flame resulting in prevention of detonation.