Disclosed is a positive electrode active material that provides an improved capacity density. Specifically disclosed is a positive electrode active material for a lithium ion battery with a layered structure represented by Lix(NiyM1-y)Oz (wherein M represents at least one element selected from a group consisting of Mn, Co, Mg, Al, Ti, Cr, Fe, Cu, and Zr; x is in the range from 0.9 to 1.2; y is in the range from 0.3 to 0.95; and z is in the range from 1.8 to 2.4), wherein, when a value obtained by dividing an average of peak intensities observed between 1420 and 1450 cm−1 and between 1470 and 1500 cm−1 by the maximum intensity of a peak appearing between 520 and 620 cm−1 in an infrared absorption spectrum obtained by FT-IR is represented by A, A satisfies the following relational formula: 0.20y−0.05≦A≦0.53y−0.06.