PURPOSE: To obtain the title complex having extremely high activity by packing carbon black into a retaining cell in a specific electrolytic cell, then electrizing the retaining cell and electro-depositing manganese oxide on the surface of carbon black particles.

CONSTITUTION: First, an electrolytic solution dissolving a manganese salt comprising manganese sulfate and an aqueous solution of sulfuric acid is packed into an electrolytic cell 1, further a carbon black suspension retaining cell 3 having a partition wall with excellent water permeability but not passing suspending carbon black particles is laid in the electrolytic cell 1 and carbon black 4 is suspended in the retaining cell 3. Then, a cathode 7 is inserted and set at the outside of the retaining cell and an anode 5 is inserted and laid at the inside of the retaining cell. Then, optionally the cathode and the anode are electrized while optionally stirring the carbon black suspension and manganese oxide is electro-deposited on the surface of the carbon black to give the objective complex.