In order to clarify the defect structure of the proton-conducting oxide, CaZr0.9In0.1O3-delta, the dc polarization characteristics of the following cell (irreversible: -)Pt,Ar + H-2(O-2) + H2O/CaZr0.9In0.1O3-delta/Ar + H-2(O2)+ H2O,Pt(1: reversible) was studied at 1173 K and a defect model for this oxide was derived from the relation between the applied voltage and the external current and its dependence on the surrounding atmosphere. Using the equilibrium constants of the defect reactions and the mobility of the oxide ion vacancy as fitting parameters, a simulation of the measured current-voltage characteristics was carried out. It was found that an excellent simulation was possible based on the proposed model. The solubility of the proton in moles per one mole of unit formula was estimated to be about 0.0089 under the condition p(H2O) = 0.01 and p(O2) = 1 at 1173 K by this calculation (p(x) is the activity of gas species, X, represented by the ratio of partial pressure of X to the standard pressure, 101,325 Pa). This value is significantly larger than what has been supposed for this oxide. Therefore, the assumption that the hydrogen solubility is very small and the excess charge of the dopant is compensated mainly by oxide ion vacancies was found as a valid first approximation only in the relatively higher temperature regions.