Negative electrode active material for electricity storage devices

Provided is a negative electrode active material for electricity storage devices, which has low operating potential and is capable of increasing the operating voltage of an electricity storage device, while having excellent cycle characteristics. A negative electrode active material for electricity storage devices, which is characterized by containing a crystal phase represented by general formula R x1 R' x2 MA y O z (wherein R represents at least one element selected from among Li, Na and K; R' represents at least one element selected from among Mg, Ca, Sr, Ba and Zn; M represents at least one element selected from among Ti, V and Nb; A represents at least one element selected from among P, Si, B and Al; and 0 ≤ x1 ≤ 6, 0 ≤ x2 ≤ 6, 0 < y ≤ 12 and 0.2 ≤ z ≤ 87, excluding the cases where x1 = 0.5 and x2 = 0 and the cases where x1 = 1.5 and x2 = 0).