A calcium phosphate cement composition kit comprising (A) a powdery agent comprising 100 parts by mass of calcium phosphate powder, and 5-50 parts by mass of a powdery apatite/collagen composite, and (B) an aqueous blending liquid; a paste-like mixture being obtained by blending the powdery agent with the aqueous blending liquid, in such a proportion that the aqueous blending liquid is 15-50 parts by mass per the total amount (100 parts by mass) of the calcium phosphate powder and the powdery apatite/collagen composite, and filled in a predetermined prosthetic site in a human body to form a hardened calcium phosphate/collagen composite.