Supercritical water gasification is suitable for gasifying biomass with high moisture content. Biomass materials are easy to decompose in supercritical water. However char and tar are also produced by polymerization which proceeds simultaneously with decomposition. Char formation in supercritical water gasification cause a critical problem because it can trigger off reactor plugging. A reactors combination was proposed to prevent char plugging and to achieve high gasification efficiency at low temperature region (around 673 K) of supercritical water condition (Ind. Eng. Chem. Res. 2004, 43, 4097-4104). To improve this reactors system, we estimated char behavior in a reactor by computational fluid dynamic calculation. Gasification experiments were also conducted which showed high gasification efficiency.