Positive electrode active material and use thereof

The positive electrode active material of the present invention is characterized in that its main component is a lithium metal oxide, and the lithium concentration at the surface portion of the primary particles that make up the active material is lower than that in the interior thereof. There will be less increase in internal resistance with a secondary cell made using this active material. The above-mentioned active material can be manufactured, for example, by bringing the raw active material into contact with a treatment liquid containing metal ions, and thereby lowering the lithium concentration at the surface portion of the primary particles that make up the raw active material.