The purpose is to provide a monomer protein effective for prevention and therapeutic treatment of bone and/or cartilage diseases. Said purpose is achieved by a monomer protein having an amino acid sequence of which cysteine contributing to dimer formation of a protein belonging to TGF-β superfamily has been replaced with another amino acid. In comparison with the corresponding dimer protein, the monomer protein has a two-fold higher activity in an osteoblast cell line to induce differentiation. Other amino acids are exemplified by serine, threonine, alanine, and valine, and preferably alanine. Said protein is prepared by using Escherichia coli, yeast, insect cells, and mammal cells that have been transformed by a plasmid having a DNA sequence capable of expression of said monomer protein.