The solubilities of Ar in molten nitrate mixtures were measured by using a modified gas probe, which injected He into molten nitrate through a nozzle, collected ascending bubbles of He-Ar gas mixture, and determined the concentration of Ar in the He-Ar gas mixture. The solubilities of Ar in molten LiNO3, NaNO3, and KNO3 are determined as a function of the temperature from 463 to 513 K, and those of LiNO3-NaNO3, NaNO3-KNO3, and LiNO3-KNO3 binary systems are determined as a function of composition at 623 K. From the Ar solubility in molten nitrate mixtures, the thermodynamical properties of molten nitrate mixtures and the interaction with Ar are investigated.