An internal combustion engine is disclosed which comprises first and second cylinder units each including at least one cylinder, an induction passage divided downstream of the throttle valve into first and second intake passages leading to first and second cylinder units, respectively, a vacuum tank held at a vacuum above that in the induction passage downstream of the throttle valve, a stop valve provided at the entrance of the second intake passage and adapted to move toward its closed position when connected to the vacuum tank, and a control circuit adapted to block the supply of fuel to the second cylinder unit and connect the vacuum tank to the stop valve thereby shifting the engine operation into a split engine mode when the engine load is below a predetermined value. The control circuit includes means for forcing the engine operation into its full cylinder mode regardless of engine load conditions before the vacuum in the vacuum tank reaches a value sufficient to move the stop valve to its fully closed position.