The purpose of this study was to estimate hard tissue formation in two types of porous columnar hydroxyapatite (HA) in order to use as a scaffold for regeneration of dentine-pulp complex. Hard tissue formation in the columnar HA scaffold with a hollow center was compared to that in the columnar HA scaffold without a hollow center. The scaffolds were immersed in hyaluronic acid sodium salt solution and were soaked in bone marrow cell suspension. They were respectively implanted into dorsal subcutis of rats for 4 weeks. Serially sectioned paraffin specimens were made and observed histologically. The scaffolds with a hollow center showed new hard tissue formation in many pores between the superficies and the wall of hollow. On the other hand, in the scaffolds without a hollow center, hard tissue formation was observed in only a few pores in the area near the external superficies. The results of this study suggested that the supply of nutrition and bioactive substance from the surrounding tissue were indispensable for differentiation of bone marrow cells and formation of new hard tissue in scaffold. A large contact area of a scaffold to the surrounding tissue may contribute to nutrition supply into the pores.