NOVELTY - A sodium-ion secondary battery consists of positive electrode (2) containing a positive electrode active material, negative electrode (3) containing negative electrode active material, separator (1) and electrolyte. The electrolyte is a nonaqueous electrolyte containing a sodium ion. The positive electrode active material includes sodium-containing transition metal oxide. The negative electrode active material includes materials (a) and (b). In a full-charge state, the ratio of sodium and transition metal atom in sodium-containing transition metal oxide is 0.3 or less.    USE - Sodium-ion secondary battery is used for electric vehicle, hybrid vehicle, domestic power supply and large-sized electric power storage apparatus for industry.    ADVANTAGE - The sodium-ion secondary battery has excellent overcharge-proof property and suppressed precipitation of sodium at the time of overcharging.    DETAILED DESCRIPTION - A sodium-ion secondary battery consists of positive electrode containing a positive electrode active material, negative electrode containing negative electrode active material, separator provided between positive electrode and negative electrode, and electrolyte. The electrolyte is a nonaqueous electrolyte containing a sodium ion. The positive electrode active material includes sodium-containing transition metal oxide which occludes and discharges a sodium ion reversibly. The negative electrode active material includes material (a) and sodium which occludes and discharges a sodium ion reversibly, and material (b) to alloy. In a full-charge state, the ratio of sodium with respect to transition metal atom in the sodium-containing transition metal oxide is 0.3 or less. The reversible and irreversible capacity of the negative electrode with respect to reversible and irreversible capacity of the positive electrode is 1 or more.    DESCRIPTION OF DRAWING(S) - The drawing shows longitudinal cross-sectional view of sodium-ion secondary battery.    Separator (1)    Positive electrode (2)    Negative electrode (3)    Battery case (10)    Container main component (12)