When PRRX1 was knocked down in an osteosarcoma cell line, which is a hyperlung metastatic strain, and the proliferative capacity was measured by the WST-8 assay, it was confirmed in vitro that the proliferation was strongly suppressed. It was also confirmed that when these cells were transplanted into mice, proliferation was suppressed as compared with the control group. In addition, when the osteosarcoma cells in which PRRX1 was knocked down were examined by wound healing assay and migration assay, the number of invading cells was reduced compared to the control group. Furthermore, osteosarcoma cells in which PRRX1 was knocked down were transplanted under the skin of mice, and 6 weeks later, the lungs of the mice were taken out and the number and size of lung metastases were compared with those of the control group. Then, it was found that the osteosarcoma cell group in which PRRX1 was knocked down had a smaller number of lung metastases.