To reduce only the reactivity of active material powder with an electrolyte while hardly reducing the capacity of a nonaqueous secondary battery.

Positive active material powder 21 of the nonaqueous secondary battery contains a lithium manganese oxide comprising lithium, manganese, and oxygen. The positive active material powder 21 has active material particles 22 in which the lithium manganese oxide is Li4Mn5O12 having spinel structure, and a covered layer 23 covering at least part of the surface of the active material particles 22. The covered layer 23 is comprised of one or more metal oxides selected from a group comprising Al2O3, ZrO2, TiO2, SiO2, Si, and C.