Negative ion generating medium and its manufacturing method

A negative ion generating medium for generating negative ions from the surface of a mother material made of aluminum or aluminum alloy. The negative ion generating medium has the mother material of aluminum or aluminum alloy covered at the surface with an anodized layer on which a rare metal separated from a rare metal solution such as zirconium salt is deposited. As the rare metal is deposited in the pores provided in the anodized layer, its negative ion generating area can be increased thus releasing a large number of negative ions. The negative ion generating medium is manufactured by electrolytically processing the mother material in an electrolyte solution of sulfuric acid doped with a rare metal salt such as lithium salt to develop the anodized layer on the surface of the mother material and deposit the rare metal on the anodized layer.