The time-dependent variation of the protein release from protein incorporated apatite cake in PBS solution was studied. When protein incorporated apatite cakes were immersed in PBS, the volume of cakes dropped for 1 day of soaking period and the shrinkage of specimen seems to be stabilized after 2 or 3 days soaking in PBS. The quantity of released protein from protein incorporated apatite cake was increased with an increase of soaking period. Around several % of protein came from the protein incorporated apatite cake in PBS for 2 weeks. The time-dependent protein release was relatively high for 1 day of soaking period, after that it slowly decreased. Compared to the decrease in volume of cake, the quantity ratio of released protein and total incorporated protein was less, which may be associated with recrystallization of the apatite hydrogel. FE-SEM observation revealed both crystals formed in air or under N2 atmosphere had similar rod like shape, and seems to be almost same in size. The change of surface appearance between 1 day and 4 days or longer soaking may be due to the dissolution of the surface layer and its recrystallization. The aggregate of apatite crystals formed in air is found to be compact, but some small cavities are observed in the specimens soaked in PBS for 4 – 11 days. The particle size of protein incorporated apatite were almost similar, independent on the soaking period. But the shape of apatite in aqueous solution is changing from square and angular shape to round shape.