A positive electrode active material is made of sodium containing oxide. The sodium containing oxide contains NaALiBMO2±α that belongs to a space group P63/mmc of a hexagonal system, where the M includes at least one of manganese (Mn) and cobalt (Co). In the NaALiBMO2±α, the composition ratio A of sodium (Na) is not less than 0.5 and not more than 1.1, the composition ratio B of lithium (Li) is larger than 0 and not more than 0.3, and the α is not less than 0 and not more than 0.3.