A method of evaluating a positive electrode active material has a density ratio-determining step of determining a ratio of an apparent density Da of the positive electrode active material to a theoretical density Db of the positive electrode active material. For example, when the positive electrode active material contains no closed space in the positive electrode active material, such as closed pores of the positive electrode active material, the ratio (Da/Db) of the apparent density Da of the positive electrode active material to the theoretical density Db of the positive electrode active material will be a value close to 1; however, the more the closed space such as the closed pores exists in the positive electrode active material, the smaller the ratio (Da/Db). Thus, the ratio (Da/Db) can serve as an indicator for measuring the degree of density of the positive electrode active material.