We report a case of polymyositis with extensive disturbance of the cardiac conduction system. A 55-year-old woman had received corticosteroids for the treatment of polymyositis for six years. She was admitted to our hospital because of palpitations and vertigo. Abnormalities on electrocardiogram (right bundlebranch block, right axis deviation, and atrioventricular block) and prolonged H-V time on His bundle electrogram revealed extensive disturbance of the cardiac conduction system. Myocardial pathology showed perivascular fibrosis and fatty infiltration. As shown is this case, patients with polymyositis often develop severe cardiac disorders. Therefore, the attention should be paid to the cardiac condition of there patients.