We performed partial oxidation, steam reforming, and CO2 reforming of CH4 to Synthesis gas, using oxidized-diamond-supported group VIII metal catalysts, to investigate the properties of oxidized diamond as a support material and to avoid carbon deposition on a metal-loaded catalyst. Nickel (5 wt %)/oxidized diamond afforded the highest CH4 conversion of 24% (CH4/O-2 = 5), giving CO and H-2 at 873 K for the partial oxidation of CH4. No carbon deposition was observed with a supported oxidized-diamond catalyst (5 wt % nickel loading level) in the partial oxidation of CH4 at > 923 K. In the steam reforming of CH4, ruthenium (5 wt %)/oxidized diamond afforded a CH4 conversion of 63% (CH4/H2O = 0.33), giving CO and H-2 at 873 K. Nickel, palladium, ruthenium, and rhodium (5 wt %)/oxidized diamond showed high catalytic activities for CO2 reforming of CH4 (CH4/CO2 = 1) at 873 K.