Insoluble dye molecules of dioctadecyl-rhodamine B at the air-water surface in an ultra-diluted gas region below 10(-16) mol/cm(2) were studied with a confocal fluorescence photon-counting microscope. We observed fluorescence photon bursts whose width, frequency, and height depended on the surface density of the dye molecules, It was found that the dye molecules prefer inhomogeneous distribution even at extremely low surface density. With autocorrelation analysis, molecular transit time was found to increase with surface density, where three fitting functions were tested to discuss the dynamic behaviour of the molecules. A nanometer-size cluster formation was suggested.