An advanced oven system, which generates superheated steam containing micro-droplets of hot water (Aqua-gas) by spraying heated water into its heating chamber, has been developed. It was expected that the micro-droplets made the heat transfer coefficient of the heating medium larger and they prevented deteriorations of foods due to drying during the heating process. The heat transfer coefficients of the Aqua-gas, superheated steam and hot air maintained at 115°C were measured changing a surface temperature of a heat flowmeter. Specific differences in the dependency of the heat transfer coefficients on the temperature at the heat transfer boundary have been found between each heating medium. Drying rate of a model food sample in each heating medium has been examined also. The drying rate of the sample in the Aqua-gas was lower, and it was shown that the micro-droplets in the Aqua-gas prevented the desiccation of the food sample. In addition, the micro-droplets in the Aqua-gas were recorded by a high speed camera. The micro-droplets were observed in the Aqua-gas, and it was supported that the micro-droplets efficiently enlarged the heat transfer coefficient of the heating medium and they prevented the drying of the food sample.