An engine control system controls an internal combustion engine so that some of a plurality of cylinders in a multi-cylinder internal combustion engine are made to rest and reduced-cylinder operation takes place. Each cylinder comprises a piston, an intake valve and an exhaust valve. In a resting cylinder during reduced-cylinder operation, when the piston moves downwards, the engine control system opens both the intake valve and the exhaust valve, and when the piston moves upwards, the engine control system opens the intake valve and closes the exhaust valve.