The present invention provides an agent which induces acceleration of hard tissue formation, acceleration of cell differentiation and increase in cellular alkaline phosphatase activity, by directly acting on the cell. Specifically, it provides a hard tissue formation promoter, a cell differentiation inducer and a cellular alkaline phosphatase activity reinforcing agent comprising, as an active ingredient, a glycosaminoglycan or a salt thereof that keeps sulfate group and has the characteristics of the following (1) and (2): (1) a basic structure is a disaccharide repeating structure consisting of a hexuronic acid residue and a glucosamine residue, (2) one or less of the position among a 2-position hydroxyl group of a hexuronic acid residue, a 6-position hydroxyl group of a glucosamine residue and a 2-position amino group of the glucosamine residue in the basic structure of the aforementioned (1) does not have the sulfate group. In this connection, it is preferable that the hexuronic acid residue is a glucuronic acid residue or an iduronic acid residue.