To provide an emulsion fuel supplying system utilizing the emulsion fuel to reduce nitrogen oxide (NOx) produced in a diesel engine, and capable of changing and controlling a ratio of the water while recycling a large amount of return emulsion fuel drained from a fuel injection valve.

The return emulsion fuel drained from the fuel injection valve 22 of the diesel engine 2 is stored in a return reservoir 31. A liquid specific gravity measuring sensor 32 is mounted on the return reservoir 31, and the ratio of the water in the return emulsion fuel is grasped on the basis of the measured specific gravity. An emulsion controller 16 determines the light gas oil, the water and an emulsifier to be added, by utilizing the obtained value to supply the same into a mixer 15 with priority.