It is an object to suppress turbo lag, while suppressing a decrease in torque in an acceleration-transient state to a supercharged state, occurring owing to a rise in required load. In a prescribed steady state, an engine compression ratio εm and ignition timing Tm are set so as to achieve a maximum thermal efficiency. In contrast, in a transient state from a non-supercharged state to a supercharged state, occurring owing to a rise in required load, the engine compression ratio is corrected to a higher compression ratio εh and concurrently the ignition timing is corrected to a retarded timing value Th, in comparison with the engine compression ratio εm and the ignition timing Tm during steady-state operation for the same load, thereby increasing exhaust energy due to a reduction in cooling loss and consequently suppressing a delay of response to a rise in supercharging pressure.