A carbonaceous material prepared by heat treating an activated carbon material below 700 °C in an alkali metal hydroxide bath has a high electrostatic capacitance, and more particularly the electrostatic capacitance per unit volume thereof is several times, or even seven times or above as high as that of a conventional activated carbon for capacitors. Thus it is possible to reduce the size of a capacitor by using this material in the production of a capacitor utilizing an electric double layer.