The present invention relates to a method of producing a positive active material for a non-aqueous electrolyte secondary battery generally referred to as a lithium-ion secondary battery and is intended to provide a method of producing a new positive active material which is superior in both discharge capacity and cycle life through improvement of a complex oxide of lithium and nickel from which a high discharge capacity can be expected. In order to attain this object, a Li-containing complex oxide is synthesized in this method by firing a mixture of a nickel oxide, in which an element M (M being at least an element selected from the group consisting of Co, Mn, Cr, Fe, and Mg) is dissolved as a solid solution, and lithium hydroxide or its hydrate. By using this as an active material in the positive electrode, it is possible to realize a non-aqueous electrolyte secondary battery which has a high capacity and a long cycle life, and which will not suffer from storage at a high temperature in a charged state.