FIG. 66 is a sectional view of a conventional capacitor. FIG. 67 is a sectional view of a pressure regulating valve used in the conventional capacitor. In FIGS. 66 and 67, the capacitor includes capacitor element 20 having hollow portion 20a. Capacitor element 20 is formed by winding a pair of positive and negative electrodes which are displaced from each other in opposite directions with a separator interposed therebetween (all unillustrated). The positive and negative electrodes are each composed of a current collector made of aluminum foil and a polarizable electrode layer formed thereon. Capacitor element 20 is structured so that the anode and the cathode can be removed from the end faces (the top and the bottom of FIG. 66).
The capacitor further includes metal case 21, which is a bottomed aluminum cylindrical case to accommodate capacitor element 20 together with an unillustrated driving electrolyte. Metal case 21 has cathode terminal 21a, which is an external-connection terminal integral with the outer bottom surface thereof. Metal case 21 further has protrusion 21b, which is integral with its inner bottom surface and inserted into hollow portion 20a of capacitor element 20. Protrusion 21b is thus inserted into hollow portion 20a of capacitor element 20, so that the cathode-side surface of capacitor element 20 in metal case 21 is mechanically and electrically joined by, for example, laser welding to the inner bottom surface of metal case 21.
The capacitor further includes sealing plate 22 made of aluminum. Sealing plate 22 has anode terminal 22a integral with its front surface and protrusion 22b, which is inserted into hollow portion 20a of capacitor element 20. The anode-side surface of capacitor element 20 is joined by, for example, laser welding to the inner surface of sealing plate 22 so as to be mechanically and electrically connected thereto. The anode-side surface of capacitor element 20 is then sealed by applying a curling process (generally called “curling”) to the opening of metal case 21 on the periphery of sealing plate 22.
The capacitor further includes pressure regulating valve 23 having gas-permeable member 24 and blocking member 25 which are disposed on electrolyte filling hole 22c of sealing plate 22. Pressure regulating valve 23 further has metal cap 26, which covers and fixes gas-permeable member 24 and blocking member 25 so as to bias blocking member 25 constantly in the closing direction. Metal cap 26 has hole 26a for communication with outside. Metal cap 26 is fixed by, for example, caulking annular projection 22d against metal cap 26. Annular projection 22d is previously disposed on sealing plate 22 in such a manner as to enclose electrolyte filling hole 22c. 
Pressure regulating valve 23 is of self-recovery type and releases the pressure to the outside when the internal pressure of the capacitor exceeds a predetermined value. Pressure regulating valve 23 allows the gas generated in the capacitor to be released to the outside so as to prevent a pressure increase in the capacitor. After the operation, pressure regulating valve 23 returns to the original state so as to keep the airtightness of the inside of the capacitor. As a result, when the gas generation causes a pressure increase in the capacitor, the capacitor can maintain its properties without exhibiting any apparent abnormalities.
Pressure regulating valve 23 functions to prevent the permeation of the driving electrolyte. Gas-permeable member 24 in close contact with sealing plate 22 is made of a material that allows the gas generated in the capacitor to pass through to the outside. This prevents the driving electrolyte from adhering to blocking member 25 and hence damaging the function of pressure regulating valve 23. Pressure regulating valve 23 further functions to prevent the driving electrolyte from leaking to the outside of the capacitor.
In the conventional capacitors having the above-described structure, pressure regulating valve 23 is fixedly attached to electrolyte filling hole 22c so as to reduce the number of locations from which the driving electrolyte might leak out. One of the prior arts related to the present application is Japanese Patent Unexamined Publication No. 2004-134632.
In the conventional capacitor, pressure regulating valve 23 is fixedly attached to electrolyte filling hole 22c so as to reduce the leakage of the driving electrolyte. However, gas-permeable member 24 and sealing plate 22 are not joined to each other and therefore the driving electrolyte is likely to leak through their boundaries. The leakage can be prevented by joining gas-permeable member 24 and sealing plate 22 by bonding or welding; however, the joining is very difficult due to the differences in their material properties.