The electric double layer capacitor (EDLC) is considered to be one of the promising systems for electric energy storage. Both the optimization of the micropore structure of the activated carbon electrode and the higher voltage operation of the EDLC are necessary for improving the energy density of the EDLC. Thus, there are already many research examples related to the former. The author focused on the latter to achieve a breakthrough in the energy density. The author will introduce the recent results of the surface modification using nitrogen monoxide and the heat-treatment above 1000°C for the activated carbon electrode.