A sodium ion secondary battery includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a separator provided between the positive electrode and the negative electrode, and an electrolyte, the positive electrode active material containing a sodium-containing transition metal oxide that reversibly intercalates and deintercalates sodium ions, in the sodium-containing transition metal oxide in a fully charged state, the ratio of sodium atoms to transition metal atoms, i.e., Na/MT, satisfying Na/MT≦0.3, and the ratio of the total Cnt of the reversible capacity and the irreversible capacity of the negative electrode to the total Cpt of the reversible capacity and the irreversible capacity of the positive electrode, i.e., Cnt/Cpt, satisfying 1≦Cnt/Cpt.