An electric double layer capacitor is disclosed which is capable of preventing transmission of an electrolytic solution vaporized in a basic cell through current collectors and capable of improving a yield. A method for preparing the electric double layer capacitor is also disclosed. The electric double layer capacitor includes a basic cell prepared by the steps of: applying a current collector onto a surface of a substrate; disposing an electrically insulating gasket element on the surface the substrate in such a manner that the current collector laterally abuts on the gasket element to define a concavity; placing a polarizable electrode impregnated with an electrolytic solution on the surface of the current collector; oppositely disposing two intermediate products prepared through the foregoing steps on a separator in such a manner that the gasket elements abut on surfaces of a peripheral portion of the separator; fusion-bonding the gasket elements together to laterally cover the separator with the resulting unified gasket; and removing the substrates from the current collectors and the gasket. The substrate has a vapor transmission rate of a predetermined one or lower with respect to the electrolytic solution vaporized at a temperature of the fusion-bonding.