A combined electrode comprises a negative electrode, a first intermediate electrode, a positive electrode and a second intermediate electrode, wherein a main body portion of the negative electrode and a main body portion of the first intermediate electrode, a connection portion of the negative electrode and a connection portion of the first intermediate electrode, main body portions of the positive electrode and main body portions of the second intermediate electrode, and a connection portion of the positive electrode and a connection portion of the second intermediate electrode are arranged in parallel to and directly facing each other, thereby increasing a facing area between the negative electrode and the first intermediate electrode and between the positive electrode and the second intermediate electrode, reducing a current loop area between the negative electrode and the first intermediate electrode and between the positive electrode and the second intermediate electrode.