Negative electrode active material for electricity storage device, and method for producing same

Disclosed is a negative electrode active material for an electricity storage device, which contains at least SnO in the composition thereof. The negative electrode active material for an electricity storage device is characterized by having a composition that contains, in mol% in terms of oxides, 70-95% of SnO and 5-30% of P 2 O 5 (excluding 70% of SnO and 30% of P 2 O 5 ).