To increase specific surface area, reduce residual sulfuric acid radical, and increase low temperature discharge characteristics by crushing manganese dioxide prepared by electrolyzing manganese sulfate solution containing sulfuric acid, treating with aqueous ammonia for deacidification, then heat treating.

Manganese sulfate solution containing sulfuric acid is electrolyzed at 70-80°C, and porous manganese dioxide with low bulk density is deposited on a positive electrode. The manganese dioxide obtained is crushed in the desirable particle size, sufficiently washed with running water, deacidified with ammonia water, and sulfuric acid radical in the electrolytic manganese dioxide are sufficiently removed. The electrolytic manganese dioxide is heated at 300-450°C to convert γ-MnO2 into β-MnO2 to use as a positive electrode material for a lithium battery. The manganese dioxide obtained is mixed with a conducting material such as acetylene black and a binder such as fluororesin, and the mixture is press-molded to form a positive mix.