A positive active material for use in nonaqueous secondary batteries, the positive active material comprising, as the crystalline structure of a main crystalline phase, a lithium-containing transition metal oxide containing manganese. The positive active material contains a minor oxide having the same oxygen arrangement as the lithium-containing transition metal oxide and having a different elemental composition and further contains tin(IV) oxide, the minor oxide and the tin(IV) oxide being contained in such a state that the presence thereof can be ascertained through diffractometry.