The invention offers a technique that selectively differentiation-induces mesenchymal stem cells, which can differentiate to cells that constitute various tissues and organs, to osteoblasts. In addition, the invention offers a technique that differentiation-induces mesenchymal stem cells to osteoblasts with a simple operation that needs only short time and that is noninvasive. The inventors have found that the switch for the differentiation induction to osteoblasts is turned on by translocating biological clock-relevant factors existing in mesenchymal stem cells from the cells' cytoplasm to the cells' nucleus. The inventors have also found that the switch can be turned on by irradiating the cells for a short time with a lightwave having a specific wavelength that is noninvasive.