PURPOSE: To improve operability on each cycle by controlling the driving of a sleeve valve, for opening/closing an intake port provided at the periphery of the lower part of a cylinder sleeve, both in two-cycle and four-cycle operation according to the change of engine speed and load.

CONSTITUTION: Plural intake ports 12 are bored at the peripheral wall, corresponding to the vicinity of the upper part of a piston head 21 positioned at the bottom dead center of a piston 2, of a cylinder sleeve 11 provided at the inner wall of a cylinder 1. The intake ports 12 are opened/closed by the rotation of a sleeve valve 3 fitted at the cylinder sleeve 11. An intake and an exhaust valves 5, 6 driven by solenoid valve actuators 51, 61 are provided at the upper part of the cylinder 1. The sleeve valve 3 is controlled in such a way as to be delayed in its opening time down to its bottom dead center at the time of engine speed being below the set engine speed lower than the cycle change- over judging engine speed N1 in two-cycle operation, and to be opened immediately before the closing of the intake valve 5 at the time of engine speed being over the engine speed N1 in four-cycle operation.