Microscopic polyangiitis is a vasculitis which primarily affects capillaries, venules or arterioles. Involvement of small and medium-sized arteries may also occur. A 70-year-old Japanese female with a fever and cough was diagnosed with pneumonia and antibiotics were administered. Her symptoms initially improved, but her fever recurred and she experienced malaise and loss of appetite. Her renal function gradually worsened and she was positive for myeloperoxidase antineutrophil cytoplasmic antibodies (MPO-ANCA). She was referred to our hospital on the suspicion of ANCA-associated glomerulonephritis. However, her depressive mental symptoms did not allow her to undergo a renal biopsy. She was clinically diagnosed with ANCA-associated glomerulonephritis, and oral corticosteroids and intravenous methylprednisolone were administered. Her symptoms and renal function were improved, but she died suddenly 15 days after admission. An autopsy disclosed approximately 700 mL bloody ascites. Coagulation adhered to the lesser curvature of the stomach, but the source of hemorrhage could not be detected macroscopically because the gastric mucosa did not show abnormal findings. The histological findings revealed that the left gastric artery showed necrotizing angiitis and rupture. In the kidneys, cellular crescents were found in approximately 10%, fibrous crescents were found in approximately 10%, sclerosis and collapse were found approximately 30% of the glomeruli, and necrotizing angiitis was observed in interlobular arteries and arterioles. From these findings, she was finally diagnosed with microscopic polyangiitis. Microscopic polyangiitis is an extremely rare cause of spontaneous intraperitoneal bleeding, but it must be carefully considered in the differential diagnosis for the appropriate management of such patients.