A 56-year-old man with idiopathic pulmonary fibrosis developed acute symptoms, including fever and cough. Chest X-ray and CT scan films revealed consolidations and ground glass opacities in the bilateral lungs suggestive of massive pneumonia and acute respiratory distress syndrome (ARDS). Microscopic examination by a transbronchial lung biopsy from the right middle lobe demonstrated numerous hyphae of aspergillus. Despite treatment with anti-fungal agents, respiratory failure rapidly progressed, and the patient died on the 21st hospital day. Postmortem examination of the lung showed multiple abscesses with hyphae of aspergillus invading into small pulmonary artery and alveoli, which were the characteristic findings of invasive pulmonary aspergillosis (IPA). In addition, diffuse alveolar damage was also present, associated with the typical features of UIP such as honeycombing in the bilateral lungs. The serum anti-influenza B virus antibody was high at 512-fold. Therefore, it was speculated that influenza B virus infection triggered superimposed aspergillus infection leading to fatal IPA associated with ARDS.