Ultraslow response, on the order of minutes, of the interfacial tension to the change in the phase-boundary potential at the interface between water and a room-temperature ionic liquid, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, has been demonstrated. This ultraslow relaxation, which is not observed at the interface between two immiscible electrolyte solutions made of molecular organic solvents, is likely to be due to the long-range and collective ordering of ions of the electrical double layer on the ionic liquid side of the interface.