A high-energy-density hybrid electrochemical capacitor (HEC) has been developed. The advanced HEC has graphitizable carbon activated with KOH (KOH-activated soft carbon) for the positive electrode instead of activated carbon, which is used in a conventional HEC or an electric double-layer capacitor (EDLC). The KOH-activated soft carbon had low electrostatic capacitance due to its small specific surface area in the initial state; however, the capacitance increased significantly by the first charging at potential over 4V versus Li/Li+ and exceeded the capacitance of activated carbon. The resulting capacitance was affected by the type of anion species and organic solvent of the electrolyte. In charge–discharge tests using various kinds of electrolytes, the KOH-activated soft carbon showed the best performance in the electrolyte of LiPF6 dissolved in a binary solvent composed of ethylene carbonate (EC) and dimethyl carbonate (DMC). The advanced HEC using KOH-activated soft carbon for the positive electrode gave about 8.5 times the energy density of the conventional EDLC.