An object of the present invention is to provide a method for generating an osteoblast that is applicable to repair of a bone defect due to various tumors, injuries, surgeries, etc. and to treatment for bone resorption typified by a periodontal disease, bone fracture, osteoporosis, etc., and that has a low risk of carcinogenesis. Provided as a means for achieving this object is a method for generating an osteoblast from a somatic cell of a mammal, the method comprising introducing Oct9 gene or an expression product thereof into the somatic cell.