Amyloidosis is caused by deposition of insoluble amyloid protein in the extracellular space of organs and tissues. The causes of amyloidosis are classified as primary, secondary, and hereditary, and symptoms develop according to which organ is involved. Cardiac amyloidosis induces cardiomyopathy and is developed by deposition of amyloid proteins in cardiac tissue. We diagnosed a patient with rhabdomyolysis and thyrotoxicosis with underlying Graves' disease 5 years ago. The patient was readmitted recently complaining of general weakness and mild dyspnea, and was diagnosed as relapsed thyrotoxicosis. An echocardiogram was performed for the evaluation of dyspnea and the findings were compatible with infiltrative cardiomyopathy due to amyloidosis. A biopsy of the abdominal subcutaneous fat and rectal mucosa was performed, and diagnosis was amyloidosis with histologic findings. The cause of heart failure was therefore cardiac amyloidosis rather than thyrotoxicosis. This case indicates the importance of evaluating the cause of heart failure in patients with thyrotoxicosis.