An electric double layer capacitor is provided with a cell, in which a pair of polarizable electrodes, which are impregnated with an electrolyte, are disposed in opposition on either side of a separator; wherein the polarizable electrodes are provided with collector electrodes, a peripheral portion of the cell is sealed by a sealing member composed of synthetic resin, and the sealing member is formed by abutting two case halves. A manufacturing method for the capacitor includes a process step of providing at least one long slit in the collector electrodes; a step of forming the case halves formed by insert molding on the collector electrodes and filling the slit, the case halves comprising a recess, one side of which extends in the lengthwise direction of the slit and, the cell being fitted into a portion inward from the slit; and a step of joining the case halves by abutting the case halves and applying ultrasonic vibration either in a perpendicular direction or a diagonal direction with respect to the lengthwise direction of the slit.