To enable the smooth switching of an operation cycle in a variable cycle engine.

In the engine capable of switching 2-cycle and 4-cycle operations according to a rotational speed and a requested load, the transient of an operation cycle is performed in the following procedure: switching from 4-cycle to 2-cycle is started after a transient operation for arranging the state of 2-cycle suitable for the initial operation following a 4-cycle expansion stroke. 2-cycle is switched to 4-cycle according to the combustion order of 4-cycle starting at a cylinder in which a 2-cycle expansion stroke is first performed after determining the switching. By this transition, the combustion can be stabilized and smooth switching can be obtained.