A man in his 50s with small cell lung cancer received amrubicin as the fourth-line therapy from August 201X-1. Serum phosphorus levels before treatment were normal at 2.9mg/dL, but grade 2 hypophosphatemia(2.1mg/dL)was observed at the beginning of the 2nd course. He underwent laryngoplasty after the 4th course. Retreatment was initiated in June 201X due to disease progression. After reinitiating treatment, the disease developed to grade 3 hypophosphatemia. As we identi- fied lower levels(1.1mg/dL)at the start of the 10th course, a pharmacist proposed oral phosphate therapy to the attending physician, which we administered. After then, the levels improved to 2.2mg/dL; thus, oral phosphate therapy was interrupted. However, because of a decline in serum phosphorus levels to grade 3, we administered the therapy again, and observed favorable improvement. For hypophosphatemia in this case, general reasons in clinical practice were not applicable; thus, amrubicin is considered to be a most possible cause.