To offer an electrical double layer capacitor which can increase the long-term reliability.

A gas permeable membrane 8 made of a water-repellent, porous polytetrafluoroethylene membrane which passes gases but is impervious to liquids is fused to the end of a small-diameter cylindrical member 9b of a sealing plug 1. There is a hole 7a passing through the center of the cylindrical member 9b and that of a tapered member 10, into which air passing through the gas permeable membrane 8 flows. In the center of a cylindrical member 9a with a diameter which is larger than that of the cylindrical member 9b, there is a hole 7b which links the hole 7a and the outside, and have a diameter which is larger than that of the hole 7a. In the hole 7b, a relief valve 2 is installed. A supporting member 3 is secured to the side wall of the hole 7b, but does not completely separate the upper and lower spaces, allowing gases to freely pass through to diffuse. A spring 4 is made to be shorter than its natural length.