To provide an emulsion fuel supply device capable of surely supplying emulsion fuel by simple control.

An emulsion fuel supply device 5 makes emulsion fuel out of light oil, water and emulsifier supplied at a prescribed ratio of light oil:water = 7:3 or 8:2 and emulsifier of 1% or the like of the entire to be supplied to a diesel engine 3. The device includes an emulsion tank 15 for storing emulsion fuel and allowing the supply of a required amount to the diesel engine 3. The emulsion tank 15 consists of a first tank 43 for receiving the supply of emulsifier and a second tank 45 which stores the emulsion fuel made by introducing mixture of light oil, water and emulsifier in the first tank 43 so that the emulsion fuel is supplied to the diesel engine 3 and which overflows mixture of more than a certain amount to the first tank 43.