PURPOSE: To easily sit a cycle switching point in an engine in which an intake port on the lower part of a cylinder or an intake valve on the cylinder top part is opened or closed according to 2 or 4 cycles, 2 or 4-cycle operation is conducted by opening/closing intake and exhaust valves and switching the timing of fuel injection.

CONSTITUTION: The 2-cycle operation of an engine is possible only when intake boost pressure is higher than exhaust pressure. Therefore, on the basis of the comparison of the detected pressures by a boost pressure sensor 26 provided on the intake pipe 22 of an engine 1 and an exhaust pressure sensor 72 provided on an exhaust passage, the engine is switched to 2 cycle and operated at the point of time where the intake boost pressure is higher in the comparison according to the operating state detected by an engine rotating speed sensor 17 and an accelerator sensor 71.