Supercritical water gasification of residue from ethanol production from Japanese cedar was studied using a graphite-supported ruthenium (Ru/G) catalyst having 5 wt % ruthenium metal loading using a batch reaction system. The complete gasification of 0.1 g of residue, which contained 72% lignin, proceeded over 0.05 g of Ru/G in water at 673 K and 0.5 g cm(-3) water density. The final composition of gaseous products was 55% methane, 40% carbon dioxide, 4% hydrogen, and <1% C-2-C-4 gases. The gasification behavior of the mixture of 0.07 g of organosolv lignin and 0.03 g of cellulose over Ru/G was almost the same as that of 0.1 g of residue in supercritical water. The Ru/G catalyst was stable in supercritical water, and 0.15 g of Ru/G maintained the complete gasification activity for 0.1 g of residue over 3 cycles.