PROBLEM TO BE SOLVED: To provide a tendon and/or ligament cell differentiation-inducing agent, and to provide a medicine containing the differentiation-inducing agent to prevent or treat diseases related to tendon or ligament.SOLUTION: The agent for inducing differentiation from cells having an ability for differentiating into tendon and/or ligament cells into the tendon and/or ligament cells contains, as an active ingredient, at least one of (A) Mkx protein, (B) a mutant which has at least one mutation selected from the group consisting of the deletion, replacement, movement, addition and insertion of one to several amino acids in the amino acid sequence of the Mkx protein and has an activity for inducing from cells having an ability for differentiating into tendon and/or ligament cells, (C) a nucleic acid encoding the protein of (A) or (B), and (D) an expression vector containing the nucleic acid (C).
