PURPOSE: To enable reliable switching to be achieved between two cycle and four cycle operations by means of a small-sized device by turning respective ring valves arranged on the peripheral parts of respective cylinders according to the driving state of the engine, for opening and closing the inlet port of respective cylinders.

CONSTITUTION: An engine 1 is operated in two-cycle, for example, in the low- speed high-load state of the engine 1. An exhaust valve 6 is opened and closed in every one rotation while intake and exhaust valve driving device 51, 61 are respectively controlled by a controller 8 to hold the intake valve 5 in its closed state. At the same time, an electromagnetic actuator 4 is controlled, thereby a ring valve 3 is driven through respective gears 32, 41 so as arrange window portions 31 to coinside with inlet ports 12 of respective cylinders. And the intake-air in an intake passage 13 is introduced into a combustion chamber from each inlet port 12. On the other hand, exhaust gas in a combustion chamber 21 is discharged to the turbine 72 of a turbocharger 7 from an exhaust line 15 through the exhaust valve 6.