A fuel supply system attached to the gasoline engine includes a first tank storing a mixture of gasoline and ethanol, a second tank storing water, a microbubble generator connected to the first and second tanks, and a fuel injection valve connected to the microbubble generator. The mixture and the water are mixed in an environment where microbubbles generated by operation of the microbubble generator are supplied. The mixed fuel thus prepared is injected, via the fuel injection valve, into a supply pipe connected to a cylinder block which is included in the gasoline engine. The mixed fuel is then mixed with the air flowing through the supply pipe to be supplied into a combustion chamber. The resulting configuration lowers the combustion temperature in the gasoline engine so as to reduce the amount of NOx contained in exhaust gas.