A 80-year-old man was admitted to our hospital because of coughing, hemosputum and dyspnea. As a chest X-ray showed infiltrates of the right lung, he was diagnosed as bacterial pneumonia and treated with antibiotics. However, after a few days, he exhibited hemoptysis and developed severe dyspnea, while laboratory findings showed rapid elevation of the serum creatinine level (5.55 mg dL). Computed tomography (CT) revealed large areas of ground glass opacity in the right lung, hence the hemoptysis was considered to be due to alveolar hemorrhage. As he had been diagnosed as chronic renal failure a few years before this admission and we also noticed that interstitial pneumonia with a slightly elevated level of C-reactive protein had existed from that time, ANCA-associated vasculitis was suspected to be the underlying pathogenesis. Accordingly, he was started on methylprednisolone pulse therapy and temporary hemodialysis resulted in improvement of dyspnea and renal function. PR3-ANCA was 12.4 EU, so he was diagnosed as PR3-ANCA-associated vasculitis. After a few days, he suddenly complained of abdominal pain, developing hypotension and anemia. Abdominal CT showed an irregular low-density mass in the right muscle, so he was diagnosed as rectus muscle hematoma. Surgery was performed and a massive hematoma was found in the rectus muscle without any ruptures of macroscopic vessels in the abdomen. Bleeding could not be stopped followed by multiple organ failure and the patient died four days postoperatively. Rectus muscle hematoma is an uncommon cause of acute abdomen, and has been reported in about 100 cases in Japan. It occurs because of a tear in epigastric vessels and is usually managed conservatively with a good prognosis, although hemodynamically unstable cases require surgery. To the best of the authors' knowledge, this is the first case of rectus muscle hematoma complicated with ANCA-associated vasculitis.