To prevent the occurrence of a sudden change in torque when supercharging pressure is not proper just after switching to two-cycle operation from four-cycle operation.

When an operation state of an engine 10 changes between an area for performing premix spark ignition four-cycle operation and an area for performing premix compressed self-ignition two-cycle operation, operation of the engine 10 is properly performed after switching an operation cycle by properly controlling actual supercharging pressure Pr before switching the operation cycle. When air quantity sucked in a cylinder 142 is kept substantially constant by adjusting the valve timing of intake-exhaust valves 132 and 134 according to a change in the supercharging pressure Pr, a fluctuation in the torque is further reduced.