To increase the size of the crystal grains, while maintaining the crystal structure suitable for a positive electrode material in the case of reacting the nickel monoxide with a lithium compound in solid phase reaction, by using polycrystalline nickel monoxide grains formed of loosely bound fine crystals as nickel monoxide.

Fine crystal for forming polycrystalline nickel monoxide grains has a gain diameter of 5 μm or smaller and a lithium-nickel complex oxide obtained by reacting polycrystalline nickel monoxide grains formed of loosely bound fine crystal and a lithium compound in solid phase reaction is preferably used for the positive electrode material. Solid phase reaction is subjected under the condition, for example, of oxygen atmosphere or aerial atmosphere at 750-850°C for 12-36 hours of burning. Polycrystalline nickel monoxide grains can be produced by heating nickel hydroxide or nickel nitrate while insulating air, or applying mechanical alloying processes to the fine crystal of nickel monoxide.