Tin-doped indium oxide nanoparticles characterized in that the average diameter of primary particles measured by observation through a transmission electron microscope is in the range of 5 to 100 nm, and that the degree of vacuum measured by the pycnometer method is in the range of 6.8 to 7.2 g/cm<SUP>3</SUP>. Preferably, the value of compact resistance after firing at 300°C in atmospheric air is less than 2O cm, and the ratio of change of compact resistance value after successive storage at room temperature for 300 hr is 10% or below.
