Isolation condensers (ICs) are important passive cooling systems in BWRs. After the Fukushima Daiichi Nuclear Power Station accident, concerns if the IC was able to restart with the inflow of hydrogen were arose. Because ICs lose heat removal ability when non-condensable gas inflow occurs, accurate evaluation of the effect is necessary. To develop analysis methods, as an initial stage, experiments and analyses considering only high-pressure steam and water were conducted. The experiment was done by an isolation condenser simulator which contains an accumulator with heaters inside, and a heat transfer tube. From the experiment, all steam was condensed at the heat transfer tube and the approximate position of complete condensation was confirmed from the temperature distribution and the observation. The experiment provided data such as temperature distribution, natural circulation flow rate, and pressure to compare with the analysis. The analyses were conducted for 4 cases of void fraction values at the heat transfer tube inlet and found that it has a high sensitivity to condensation. The reason is estimated to be the difference in inflow velocity that strongly depends on the void fraction even if the mass flow rate is constant. And the initial condition of the liquid film also affected condensation process. Heat removal at the section before the heat transfer tube should be considered to adjust void fraction at the inlet of heat transfer tube.