Abstract: A spark-ignition engine that has a cylinder and a crank connected to the piston in the cylinder and which converts the reciprocating motion of the piston to rotary motion via the crank, characterized in that said engine further includes:an angle sensor for measuring the crank angle;a combustion sensor for measuring the burned mass fraction of a fuel in the combustion chamber;a computing unit for calculating the ignition timing of the spark-ignition engine on the basis of the measured crank angle and burned mass fraction by the following equation: Y=aX+b(where Y is the ignition timing expressed by the crank angle before top dead center; X is the difference between the crank angle at a reference burned mass fraction of the fuel injected into the cylinder and the crank angle at a burned mass fraction in the progress of combustion; a and b are constants determined by the characteristics of the spark-ignition engine); anda control unit for controlling the ignition timing of the spark ignition engine on the basis o