A method of producing a positive electrode active material, comprising the steps of: preparing a solution by dissolving, in a solvent, respective predetermined amounts of a lithium source, a M source, a phosphorus source and a X source necessary for forming a positive electrode active material represented by the following general formula (1) having an olivine structure; gelating the obtained solution by addition of a cyclic ether; and calcinating the generated gel to obtain a carbon-coated lithium-containing composite oxide, wherein the positive electrode active material is represented by the general formula (1): LixMyP1-zXzO4  (1) wherein M is at least one element selected from the group consisting of Fe, Ni, Mn, Zr, Sn, Al and Y, X is at least one selected from the group consisting of Si and Al, and 0