Abstract: In order to prevent the increase of NOx due to an excessively high combustion temperature and the increase of HC and CO due to an excessively low combustion temperature, by feeding an emulsion of a fuel oil and water having a water content suitable for the combustion state in cylinders to a Diese engine, there is proposed: A combustion parameter sensor (3) for detecting combustion parameters inside cylinders such as an internal pressure of the cylinder, an engine knock, an exhaust gas, etc. and a water charging quantity judgement/regulation circuit (2) for setting a water charging quantity to a fuel are disposed in a Diesel engine (1). A water charging quantity controller (4) feeds water to an emulsion formation apparatus (5) in a quantity corresponding to a water charging signal from the water charging quantity judgement/regulation circuit (2).