To prevent undesired premature ignition and knocking due to high temperature of gas in a cylinder immediately after switching to two cycle operation from four cycle operation.

When switching to premix compression self ignition two cycle operation from premix spark ignition four cycle operation is carried out, an exhaust valve 134 is opened for a long time or twice in a transition period to suck exhaust gas discharged to an exhaust port 16 once in a cylinder 142 again. Since temperature of exhaust gas discharged once and sucked in again via the exhaust valve 134 drops due to heat exchange, gas temperature in the cylinder during premix compression self ignition two cycle operation drops and undesired premature ignition and knocking do not occur.