A 77-year-old woman was referred to our department with hemoptysis. Microscopic polyangiitis (MPA) with resultant alveolar hemorrhage was diagnosed because of diffuse infiltrate of the right lung, proteinurea, renal dysfunction and the presence of MPO-ANCA. The disease responded well to corticosteroid therapy. She was discharged, but as corticosteroid was gradually tapered, an irregularly-shaped nodule appeared in the right upper lung field within 2 weeks. She was re-admitted because the nodule increased in size with cavity formation in spite of the administration of antibacterial agent. Pulmonary aspergillosis was diagnosed, since bronchial washing and transbronchial lung biopsy revealed the presence of Aspergillus fumigatus. Serum beta-D-glucan was decreased and the cavity was reduced in size, responding to the treatment with micafungin. However, she died later of systemic infection by a herpesvirus. We report this case because of the interesting course of pulmonary aspergillosis that subacutely formed a cavity.