PURPOSE: To increase the rigidity of a case and elongate the service life without increasing the thickness of the case by increasing the rigidity of the case.

CONSTITUTION: A flat capacitor element 1 is made by winding an aluminum foil, which has a + pole and a - pole electrochemically treated and insulating paper, and this capacitor element 1 is inserted in a flat metallic case 2 together with electrolyte. Three grooves 5 each are made at equal intervals in parallel with the axial direction at the plane parts on both its sides in advance in the case 2. A plug plate 3 is fit in the opening of the case 2, and the margin of the case 2 is folded in side of it so as to prevent the electrolyte from leaking. A pair of electrodes 4 are attached through to the plug plate 3, and they are connected to the inside capacitor 1.