A flexible secondary battery includes an electrode stack structure. The electrode stack structure includes a first electrode layer including a first metal current collector, a second electrode layer including a second metal current collector, an isolation layer between the first electrode layer and the second electrode layer, connection tabs respectively extended from an end portion of the first metal current collector at a first end portion of the first electrode layer and an end portion of the second metal current collector at a first end portion of the second electrode layer; and a fixing element which fixes the end portions of the first and second metal current collectors only at a first end portion of the electrode stack structure. Second end portions of the first and second electrode layers opposite to the first end portions thereof are movable.