This study constitutes a part of attempt to develop a clean and high efficiency combustion system utilizing reactions in supercritical water. This study is aimed at establishing the measuring techniques of transport properties and observing techniques of reaction zone in supercritical water. First, the refractive index of water was measured because previous values were not available in supercritical region. The measurements were made at the temperatures from ambient temperature to 420°C and pressures from atmospheric pressure to 27MPa. The results show that the refractive index is constant in the experimental conditions within experimental errors. Next, the mass diffusivity of gas in supercritical water was measured. Nitrogen was used in this experiment to avoid the effects of reaction. The mass diffusivity of nitrogen in supercritical water was calculated from the fringe shift. The results show that the activation energy of the diffusion is as large as those of chemical reactions.