With the object of providing a positive electrode active material for lithium battery that can increase the filling density, can increase the output characteristics, and furthermore, with a small voltage decrease during conservation at high temperature in a charged state, a positive electrode active material for lithium battery is proposed, containing a spinel type (Fd3-m) lithium transition metal oxide represented by general formula Li1+xM2−xO4−δ (where M represents a transition metal including Mn, Al and Mg, x represents 0.01 to 0.08 and 0≦δ) and a boron compound, the inter-the atomic distance Li—O of the spinel type lithium transition metal oxide being 1.971 Å to 2.006 Å, and the amount of magnetic substance measured for the positive electrode active material for lithium battery being 600 ppb or less.