Fanconi syndrome is characterized by some absorptive defects of proximal renal tubules. It is observed either primarily, or secondarily to various diseases. A case of multiple myeloma complicated by Fanconi syndrome is reported. The patient was a 62-year-old female. The serum total protein was elevated, and monoclonal IgG-kappa protein was found on serum immunoelectrophoresis. Urinary Bence Jones protein was positive. Bone marrow aspiration disclosed increased dysplastic plasma cells, which led to the diagnosis of multiple myeloma. A diagnosis of Fanconi syndrome was based on renal glycosuria, hypophosphatemia, hypokalemia, and panaminoaciduria due to abnormal excretion from the kidney, and metabolic acidosis. After chemotherapy for multiple myeloma, serum IgG level and urine sugar decreased, and serum potassium level was corrected.