A sodium ion secondary battery having far superior potential stability during discharge when repeatedly charging and discharging, and a negative electrode active material capable of being efficiently doped and dedoped with sodium ions used therefor are provided. The sodium ion secondary battery according to the present invention includes a positive electrode containing a positive electrode active material capable of being doped and dedoped with sodium ions, a negative electrode containing a negative electrode active material containing, as a sole component or as a main component, a glassy carbonaceous material capable of being doped and dedoped with sodium ions, and an electrolyte containing sodium ions. Further, the negative electrode active material for a non-aqueous electrolyte sodium ion secondary battery according to the present invention includes a glassy carbonaceous material as a sole component or as a main component.