Effect of added α-tricalcium phosphate (α-TCP) and β-TCP was investigated to understand the setting reaction of apatite cement consisting of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA). Addition of TCP delayed the initial setting time because TCP was not involved in the initial setting reaction and resulted in the decreased initial mechanical strength. After the initial setting of the cement due to the conversion of TTCP and DCPA into apatite, α-TCP dissolved to supply calcium and phosphate ions and they were consumed for crystal growth of apatite. Therefore, mechanical strength of the apatite cement containing α-TCP was increased. In contrast, added β-TCP showed no reactivity in the cement and thus result in the decreased mechanical strength.