To provide a fuel leakage diagnostic device for a fuel supply system not releasing the evaporated fuel after the termination of the diagnosis of the fuel leakage.

This fuel leakage diagnostic device 11 of the fuel supply system wherein the fuel tank 1 accommodating the fuel 2 is pressurized to diagnose the presence or absence of the fuel leakage on the basis of the variation of the pressure in the fuel tank 1, comprises an air container 12 mounted in the fuel tank 1 and increasing and decreasing its volume in accordance with the amount of air accommodated inside, and an air supplying/releasing means 13 for supplying the air outside of the fuel tank 1 into the air container 12 by driving a pump 6, and releasing the air supplied in the air container 12 to the outside of the fuel tank 1, and the inside of the fuel tank 1 is pressurized by expanding the volume of the air container 12 by supplying the air into the air container 12 by the air supplying/releasing means 13.