The secretion of collagen by osteoblasts was observed by scanning electron microscopy (SEM). Osteoblast-like cells were cultured on a CO3apatite-collagen sponge reinforced with a porous HAp frame. After three days' incubation, a number of newly created matrix fibrils, forming a network structure, were observed at the cell surface. SEM also showed that osteoblasts secreted collagen fibrils from their membrane, and that the collagen fibrils were twisted together. When collagen in an aqueous sol solution was sprayed onto the extremely smooth surface of a mica plate to support the secretion of osteoblasts, a collagen network structure could be clearly observed with atomic force microscopy (AFM). With this in vitro phenomenon, we could confirm the formation of collagen network structure without biological function. Therefore, it was suggested that the CO3apatite-collagen sponge used in this study is a favorable scaffold biomaterial, on which osteoblasts could produce the unmistakable, characteristic extracellular matrix for mineralization. For therapeutic use of hard tissue biomaterials, collagen formation as an extracellular matrix (ECM) is very important because mineralization is subsequent to ECM.