It was early shown that Ti metal spontaneously bonds to living bone through an apatite layer formed on its surface in the living body, when it was subjected to NaOH and heat treatment to form a sodium titanate on its surface. This kind of bioactive Ti metal was applied to artificial hip joint and already clinically used in Japan. During the study of fabrication process of the hip joint, it was found that sodium content of the surface layer of the Ti metal is largely varied with washing condition after the NaOH treatment, and that apatite-forming ability of the NaOH- and heat-treated Ti metal is liable to decrease in humid environment for a long period. In the present study, the sodium content of the surface layer of the Ti metal was systematically changed by water or HCl treatment after NaOH treatment. Effect of the sodium content of the surface layer on apatite-forming ability in a simulated body fluid and its stability in humid environment of the NaOH- and heat-treated Ti metal were investigated. As a result, it was found that the NaOH- and heat-treated Ti metal gives high and stable apatite-forming ability, when the Ti metal was treated with water after the NaOH treatment to remove partially sodium ions in the surface layer, and subjected to heat treatment.