Electric double-layer capacitors based on the charge storage at the interface between a high specific surface area activated carbon electrode and an electrolyte solution have been developed, and are widely used as maintenance-free power source for IC memories and microcomputers. Following the production of small size capacitor of the capacitance up to 1F, middle size capacitor having the capacitance up to 100F were produced and applied to power sources combined with solar cell in traffic signal and so on. Recently, large size capacitors of the capacitance about 1500F are being developed for large power sources such as in HEV. However, the energy density of electric double-layer capacitors is smaller than that of batteries such as the lithium ion battery. To solve this problem, new electrolytes, electrode materials and the construction of the cells were developed. As a result, the energy density of available large device (1000-2000F) was enhanced to 6 Wh/l at power density of 800W/1 and expected to be larger in near future.