A case of oncogenic osteomalacia is reported in a 71-year-old man who presented with bone pain, muscle weakness, and severe hypophosphatemia. The tumor which was localized in the left lower mandible was not detected by tomodensitometry, resonance magnetic imaging, and (111)IN-octreotide scintigraphy, but was easily localized by F-18 fluorodeoxyglucose PET/CT SCAN (F-18 FDG PET/CT SCAN). To our knowledge, the value of this technique for detecting tumors in oncogenic osteomalacia has never been reported. Secondly, this case provided an opportunity for confirming the usefulness of serum fibroblast growth factor 23 (FGF23) measurement for the diagnosis and follow-up. We conclude that FGF23 measurements combined with F-18 FDG PET/CT SCAN were decisive tools in a case of oncogenic osteomalacia and are likely to be of considerable importance for facilitating early diagnosis and follow-up in the future.