The electrical conductivity and the oxygen release and uptake of sintered indium tin oxide (ITO) upon heating and cooling under the condition that P(O2)/P*=4.9 × 10-4 (P* = atmospheric pressure), when P(O2) was approximately 50Pa, were simultaneously measured by using a closed-system oxygen-gas analyzer. A large increasing in the electrical conductivity upon the oxygen release detected in 1130<T<1273K was observed. Quantitative relationship between the number of oxygen atom released from ITO and the conduction electron density in ITO was studied. The release of one oxygen atom agreed with the generation of two conduction electrons, i.e., the efficiency-carrier-generation for the oxygen release from the ITO was almost one. The oxygen atom released must be originally at the quasi-anion site in the C-type rare-earth lattice, i.e., the 16c site in the space group of Ia3, as an interstitial excess oxygen, Oi″. It was concluded that the mechanism of conduction carrier generation and compensation upon SnO2 doping into In2O3 can be expressed by the defect equation, 2SnO2→2SnIn +2(1-z) e′+zOi″+3Oox+(1-z)/2O2, and that the oxygen release bringing the increase in the electrical conductivity can be expressed by the equation, Oi″→ 1/2O2+2e′.