Tumor hypoxia is considered a potential therapeutic problem because it reduces the effects of radiation therapy. Clinical experience has shown that long-term tumor oxygenation cannot be achieved with oxygen inhalation, but the mechanisms behind this phenomenon remain unknown. In this study, we designed an optical system for evaluating spatiotem-poral changes in tissue oxygen tension (pO 2) by phosphorescence quenching. The system can measure continuous changes in pO 2 at a fixed point and can also perform two-dimensional mapping of pO 2 in any part of the tumor tissue. We implanted tumor tissue in a dorsal skinfold chamber of C57BL/6 mice and observed tumor growth. After the tumor attained a diameter of 2 mm, the mice received oxygen inhalation and pO 2 was measured. Tumor pO 2 increased after inhalation but the oxygen level was not maintained despite continuous inhalation of pure oxygen; the tumor returned to a hypoxic state. These results mimic the clinical experience of oxygen inhalation treatment in radiation therapy. Our system reproduces the repeat hypoxic phenomenon in a murine tumor model and can be used to determine the mechanisms of oxygen metabolism in tumors.