Carbonate apatite (CO3Ap) is expected to be an ideal bone substitute since it can harmonize with the bone remodeling cycle. The aim of this study is to fabricate a CO3Ap bone substitute from gypsum (calcium sulfate, CaSO4·2H2O) hardening bodies based on dissolution-precipitation reaction. Calcium sulfate hemihydrate mixed with water at a water-to-powder ratio of 0.5 was packed in a split stainless mold and kept at room temperature for 24 hours to obtain set CaSO4·2H2O. The set CaSO4·2H2O was hydrothermally treated in the presence of disodium hydrogen phosphate (Na2HPO4) and sodium hydrogen carbonate (NaHCO3). The results of powder X-ray diffraction and Fourier transform infrared spectroscopy indicated that CO3Ap block could be fabricated from the set CaSO4·2H2O block by hydrothermal treatment with Na2HPO4 and NaHCO3. When the treatment temperature was increased, the conversion rate to CO3Ap increased. However, the carbonate content decreased with increasing treatment temperature.