An object of the present invention is to provide a non-aqueous electrolyte secondary cell having excellent resistance to deterioration, excellent self-extinguishability or incombustibility, and excellent discharge properties at low temperatures, a deterioration inhibitor for a non-aqueous electrolyte secondary cell which can improve resistance to deterioration, and an additive for a non-aqueous electrolyte which can improve self-extinguishability or incombustibility. The non-aqueous electrolyte secondary cell has a positive electrode, a negative electrode, and, in a first aspect, a non-aqueous electrolyte containing at least 2% by volume and less than 20% by volume of a phosphazene derivative, and a supporting salt, and, in a second aspect, a non-aqueous electrolyte containing at least 20% by volume of the phosphazene derivative, and the supporting salt. The deterioration inhibitor for a non-aqueous electrolyte secondary cell and the additive for a non-aqueous electrolyte secondary cell are added to the non-aqueous electrolyte of the non-aqueous electrolyte secondary cell and contain at least the phosphazene derivative.