A 28-year-old man with a history of mental retardation was admitted to our hospital because of dyspnea, cough and high fever. His SpO2 level at room-environmental conditions was in the eighties, and his chest radiograph showed diffuse infiltrates in both lungs. He was diagnosed as suffering from influenza A by a rapid influenza virus antigen test. The echocardiogram showed no evidence of left cardiac failure; therefore, his symptoms were consistent with acute respiratory distress syndrome (ARDS). Oseltamivir was started, and antibiotics were also given because of the possibility of secondary bacterial infection. Due to respiratory failure and low blood pressure, which suggested septic shock, intensive treatments including mechanical ventilation were performed. Corticosteroid therapy was started for ARDS and sepsis, and these therapies improved his respiratory condition. Polymerase chain reaction of his pharyngeal swab revealed that he had influenza A (H1N1). This is the first case of ARDS following infection by influenza A (H1N1) virus in Japan.