To generate the output according to an running condition and improve fuel consumption by forming a combined engine of a four-cycle engine and an eight-cycle engine, and switching it to the four-cycle engine or the eight-cycle engine according to its operating condition.

An engine is provided with an intake valve 1 for exclusive use of air-fuel mixture, an intake valve 2 for exclusive use of air opened during the first and second air intake stroke, and an exhaust valve 3. It is operated, according to an engine operating conditions, as a four-cycle engine [intake stroke → compression (ignition) stroke → expansion stroke (combustion and explosion stroke) → exhaust stroke], or eight-cycle gasoline engine [first intake stroke (intake stroke of air-fuel mixture) → compression stroke (ignition) →expansion stroke (combustion and explosion stroke) → first exhaust stroke →second intake stroke (first air intake stroke) → second exhaust stroke →third intake stroke (second air intake stroke) → third exhaust stroke].