The main purpose of the present invention is to provide: a negative electrode active material which is capable of increasing the energy density of a battery, while increasing the safety of the battery; and a metal ion battery which uses the negative electrode active material. The present invention provides: a negative electrode active material which contains an alunite group material that is capable of intercalating and deintercalating ions of at least one metal element selected from the group consisting of alkali metal elements and alkaline earth metal elements; and a metal ion battery, which has a positive electrode, a negative electrode and an electrolyte that conducts metal ions and is filled between the positive electrode and the negative electrode, and wherein the negative electrode contains the negative electrode active material.