To obtain a positive electrode active material for a lithium cell by oxidizing a transition metal hydroxide with ozone in a soln. containing a lithium salt.

This Li-contg. transition metal oxide is expressed by HxLiyMO2 and is produced by oxidizing a hydroxide expressed by M(OH)2 with ozone in a soln, containing a Li salt such as LiOH at ≤100°C. A hydroxide expressed by Ni(OH)2 containing Co by 0.02 to 0.9 atomic ratio of Co/(Ni+Co) is preferable because it makes diffusion of Li ions easy in the charge-discharge reaction, promotes production of LiNiO2 and gives uniform discharge characteristics. As for the Li-contg. transition metal oxide, LiCoO2, LiNiO2, LiMnO, α-LiFeO2r, LiCrO2, LiVO2 or the like is used. In the above formula, x, y satisfy 0<x≤2 and 0≤y≤2 and 1<(x+y)≤2, and M is a transition metal element such as Co, Ni, Mn, Fe, Cr, V and Ti.