A lithium-ion secondary battery comprising a positive electrode and a negative electrode is provided. The positive electrode comprises as a positive electrode active material a lithium transition metal composite oxide having a layered structure. The composite oxide contains as its metal components at least one species of Ni, Co and Mn as well as W and Ca. The composite oxide contains 0.26 mol % or more, but 5 mol % or less of W and Ca combined when all the metal elements contained in the oxide excluding lithium account for a total of 100 mol %, with the ratio (mW/mCa) of the number of moles of W contained, mW, to the number of moles of Ca contained, mCa, being 2.0 or larger, but 50 or smaller.