A layer-structured crystal CaSi2 was found to be oxidized with CoCl2 aqueous solutions. The CO2+ ions of the solution were reduced to metal at the edge surface of the crystals, while the interlayer calcium atoms of the crystals were removed as cations into the solution. A large amount of hydrogen was evolved during the reaction, which was attributed to a reduction product of water with a reactive silicon network generated by the deintercalation of calcium from CaSi2. The saturation magnetization of the cobalt metal deposited on the surface increased as the reaction proceeded, approaching that of the bulk metal. The oxidation mechanism of the CaSi2 is discussed on the basis of the chemical analysis data.