Adult Fanconi syndrome is characterized by variable abnormalities caused by renal proximal transport defects, resulting in glycosuria, aminoaciduria, bicarbonaturia, uricosuria and phosphaturia. A 57-year-old man with kappa-light chain multiple myeloma, undergoing chemotherapy with prednisolone and melphalan for 17 month, was admitted with spontaneous femoral neck fracture and was consulted due to polyuria and refractory metabolic acidosis immediately after hemiarthroplasty. The laboratory values showed normal anion gap metabolic acidosis with normal urinary anion gap, hypokalemia, hypouricemia, hypophosphatemia at the time of consultation. After partial correction of acidemia, the fractional excretion of HCO3- was 11.9%, it was interpreted as proximal renal tubular acidosis. 24-hour urine collection showed increased level of excretion for most aminoacids. Diffuse osteopenia and multiple compression fractures on spine were detected on radiological examinations. Also, osteoporosis and osteomalacia was suggested during his clinical course. After the diagnosis of Fanconi syndrome was made, treatment was started with sodium bicarbonate, potassium citrate, calcitriol, calcium carbonate along with phosphate rich diet. Laboratory abnormalities were corrected and refractory multiple bone pain was ameliorated with these treatment.