Todorokite type Mg x MnO2·yH2O which possesses 3×3 large tunnel structure was prepared through hydrothermal treatment of layer manganese dioxide, buserite, at 120–200°C. In non-aqueous Li and Mg cells, electrochemical insertion of Li+ and Mg2+ into the todorokite products occurred utilizing the preferable large tunnel for their diffusion. By heat-treatment up to 300°C, cycle performances in Li cell were improved by removing water molecules from the todorokite tunnel.