A lithium ion secondary battery in which at least one step portion is formed on an upper surface of a cap plate, and at least one step portion is formed on a lower or first surface of a battery part opposite to the upper or first surface of the cap plate so that the at least two step portions are complementary. The complementary step portions of the cap plate of the bare cell and the battery part provided at an upper part of the cap plate result in easy coupling of the battery part to the bare cell and stable maintenance of the coupling between the battery part and the bare cell. The coupling structure results in greater ease in performing subsequent manufacturing processes and protects the bare cells coupled to the battery part from dislodging.