An apparatus for supplying mixed fuel includes a water container storing water; a fuel container storing fuel, including at least gaseous fuel, the fuel container being coupled to the water container by a pressure pipe for transferring internal pressure of the fuel container to the water container; and a mixer coupled to the water container through a water supplying pipe and coupled to the fuel container through a fuel supplying pipe, the mixer outputting mixed fuel through a mixed fuel supplying pipe. A water supplying unit includes a water container storing water and connected to a water supplying pipe, the water container having an outlet; and a pressure applying unit coupled to the water container for applying gas pressure to an inside of the water container to discharge water from the water container through the outlet.