PURPOSE: To properly damp output of an engine by limiting the flow rate of exhaust gas passing through a turbo-supercharger when the engine speed exceeds a first engine RPM limit value, and limiting the fuel flow rate when the engine speed exceeds a second engine RPM limit value.

CONSTITUTION: In a spark ignition engine using natural gas as fuel, the fuel is supplied through a fuel supply pipe 34 in which a fuel control valve 38 is provided, and air compressed by a turbo-supercharger 12 is supplied through an air manifold 22. In an exhaust gas system, an exhaust gas bypass valve 30 is provided. In the exhaust gas system, an exhaust bypass conduit 28 in which the exhaust gas bypass valve 30 is provided, is provided so as to bypass an exhaust gas inlet 26 of the turbo-supercharger 12. In this engine, when the engine speed exceeds a first engine RPM limit value, the exhaust gas flow rate passing through the supercharger 12 is limited by the exhaust gas bypass valve 30, and when the engine speed exceeds a second engine RPM limit value, the fuel flow rate is limited by the fuel control valve 38.