Diffuse alveolar hemorrhage (DAH) is a life-threatening complication that occurs in association with various diseases including coagulation disorders. In rare cases, it is caused by hemophilia. A 48-year-old man was admitted to our hospital for a third time due to DAH. Although the cause of DAH could not be identified by bronchoscopy or laboratory tests, a good response to corticosteroids suggested idiopathic DAH with pulmonary capillaritis. The patient was diagnosed with hemophilia B based on the results of a detailed inquiry, a mildly prolonged activated partial thromboplastin time, and low factor IX activity. Hemophilia may be an underlying factor that exacerbates the bleeding of patients with DAH, even when they show a good response to corticosteroids.