A positive electrode active material for a lithium battery is provided which offers high-temperature storage property and battery properties and comprises manganese dioxide having a crystal structure that exhibits an X-ray diffraction pattern in which peaks having a peak intensity ratio of not less than 0.05 with respect to the peak at 2θ=37.3°±0.5° are present at positions of 2θ=18.2°±0.5°, 19.5°±0.5°, 28.6°±0.5°, 56.6°±0.5°, 59.1°±0.5°, and 65.1°±0.5°, or further at 2θ=22.0°±1.0°, the X-ray diffraction pattern being obtained using a powder X-ray diffraction device (XRD) that uses CuKα1 rays.