A method of efficiently removing active oxy-hydrogens (e.g., existing as hetero element-containing functional groups such as COOH, CHO, and OH) present in a carbon material at a relative low temperature. The invention also provides a carbon-activating material adapted for use in a polarizable electrode typically used in an electrical double-layer capacitor. The method of removing residual active oxy-hydrogens in the carbon material starts with mixing the carbon material and a transition metal or a transition metal compound. The resulting mixture is thermally processed within a stream of a reducing gas. Preferably, the transition metal or transition metal compound is removed from the thermally processed mixture.