Artificial bone, i.e. implant material, is widely used for reconstruction of damaged bone tissue. Implant material is required to show high biological affinity to the surrounding living tissue. Artificial materials are generally encapsulated by fibrous collagen tissues to be isolated from surrounding bone, when implanted in bony defects. However, some special types of ceramics are known to show ability of direct bone-bonding, i.e. bioactivity. Previous studies reported that the essential requirement for materials to exhibit the bioactivity is formation of bone-like apatite on their surfaces in body environment. The same type of apatite formation can be observed in a simulated body fluid (SBF) that mimics ion concentrations of human extracellular fluid. The apatite formation is triggered by specific functional groups such as Si-OH, Ti-OH, Zr-OH, Ta-OH, which can induce heterogeneous nucleation on the surface of materials. These findings bring us an idea that even bioinert metals show bioactivity, if such functional groups are formed on their surfaces with gradient structure. As a novel kind of surface treatment, the alkali treatment has been developed to induce bioactivity on various kinds of metal such as titanium, tantalum, zirconium and molybdenum. Bioactive surface layer composed of metal hydroxide is formed by the treatment and it takes a graded structure toward the interior of the substrates. Bioactive materials with graded structure are useful as artificial bones with ability of tight bone-bonding.