PURPOSE:To form the title granular manganese dioxide by electrolyzing an aq. soln. of manganese sulfate acidified with sulfuric acid while periodically reversing the polarity of an anode and a cathode and agitating the electrolyte. CONSTITUTION:The electrolyte consisting of sulfuric acid-acidified aq. soln. of manganese sulfate (contg. about 10-60g/l bivalent manganese) is supplied to an electrolytic cell 1 at a high speed of >=0.1m/sec with respect to the surface of an electrode, the electrolyte is agitated, and the polarity of the anode and cathode is reversed in a cycle of <=60min, and electrolysis is carried out. As a result, granular manganese dioxide is formed in the electrolyte. The discharged liq. after electrolysis is sent to a receiver tank 2, the formed manganese dioxide is settled therein, and the electrolyte is circulated to the electrolytic cell 1 by a pump 3. The slurry is drawn off from the lower part of the receiver tank 2, and the granular manganese dioxide is separated from the liq. by a filter 4 and taken out. By this method, granular manganese dioxide is continuously and efficiently formed.