A single nanodroplet of n-decane in a surfactant-free n-decane-in-water emulsion was detected by observing fluorescence emitted from C540 dye molecules dissolved in the droplet on the basis of confocal fluorescence microscopy. The fluorescence intensity was found to decrease when a dye-free n-decane-in-water emulsion was added, because the dye molecules are transferred to the dye-free droplets. Insensitive change of the rate of the dye transfer with the amount of the dye-free emulsion added to the dye-containing one is explained in such a manner that the permeation of the dye molecules across the droplet-water interface is the rate-determining step. The rate-decrease with the diameter of the droplet reveals that the permeability and solubility of the dye molecules into the droplet tends to increase with the droplet diameter.