In this study, the effectiveness and mechanism of combination therapy of 5-aminolevulinic acid-based photodynamic therapy (ALA-PDT) with simultaneous hyperthermia (HT) on human-derived osteosarcoma in vivo were examined. A tumor model was prepared by subcutaneously implanting human osteosarcoma into nude mice and local injection of ALA was selected as the administration route. This study demonstrated that both ALA-PDT and the combination of ALA-PDT with HT exhibited significant inhibitory effects on tumor growth. In the group with high total energy, the growth inhibition rates of tumor were 52.3% in ALA-PDT, and 27.3% in ALA-PDT with simultaneous HT (PDT+HT), and the synergetic index was 1.76, demonstrating that the inhibitory effect on tumor growth in ALA-PDT was significantly increased by simultaneous use of HT. In the histological findings, after ALA-PDT, necrosis was observed in the area from the surface to a depth of 2 mm, and only a slight effect was confirmed in deeper layers. On the other hand, after PDT+HT, necrosis was observed in layers even deeper than 2 mm below the surface. Furthermore, based on the immunohistochemical findings of the expression of carbonic anhydrase IX, while weak expression of CAIX was observed after ALA-PDT in an area relatively close to the surface, a positive area extended to the deeper layers after PDT+HT compared with ALA-PDT. In conclusion, PDT+HT demonstrated a significant inhibitory effect on tumor growth of human-derived osteosarcoma, and a synergistic interaction of simultaneous HT. This suggests the possibility that ALA-PDT is useful, not only for the treatment of superficial tumors but also for deep-seated mesenchymal tumors, such as osteosarcoma. Furthermore, the mechanism of the synergistic interaction suggested that ALA-PDT was effective in the deep area of tumors due to the increase of blood flow by mild hyperthermia.