Biomineralization on porous polyester membranes was examined using an improved alternate soaking process (ASP). The effect of ion migration for the formation of hydroxyapatite (HAp) was shown to be crucial. Ion migration was improved by reducing the surface tension by mixing ethanol into an aqueous solution. The resulting hybrid materials were evaluated in terms of calcium content; structure using scanning electron microscopy (SEM), X-ray diffraction (XRD), and infrared spectroscopy (IR); and protein adsorption. The amount of formed HAp was controlled by the number of ASP cycles and also through the ethanol content of the mixed solvent. As the formation of HAp increased, the formed structure could be verified using SEM, IR, and XRD. Protein adsorption was investigated using albumin, gamma-globulin, and fibrinogen, and the amount of adsorbed protein was well-correlated with that of the formed HAp. This result shows that the total amount of the adsorbed proteins can be regulated by the HAp content. In summary, a tunable protein reservoir was formed on a porous polyester membrane.