The present invention provides a cylindrical secondary electrode including a first electrode plate, a second electrode plate, a separator interposed between the first electrode plate and the second electrode plate, and a case having a space for receiving the electrode assembly, wherein the cylindrical secondary electrode may include a core element having a space at the center thereof and having a tubular shape with a circular cross section, which is inserted into a space at the center of an electrode assembly and has at least one slit including a bent portion.