We report a case of a 73-year-old man with pulmonary embolism due to idiopathic thrombosis of the inferior vena cava. He was referred to our hospital because of a fever and cough of 2 weeks' duration despite treatment with an oral antibiotic. Chest radiography on the first visit showed an infiltrate in the right middle lung field. He was diagnosed as having pneumonia and admitted to our hospital for treatment. Following administration of intravenous antibiotics, his symptoms disappeared and the chest radiography findings improved. The abdominal CT obtained in an attempt to visualize the cause of liver dysfunction serendipitously revealed thrombosis of the inferior vena cava, which was suspected to have caused the pulmonary embolism. A subsequent lung perfusion scan revealed marked perfusion defects in the right middle and lower lobes. Chest CT revealed an embolus located in the right pulmonary artery. Since thrombolytic therapy was not effective, the placement of a filter in the inferior vena cava was performed to prevent the recurrence of pulmonary embolism. The patient has been asymptomatic without recurrence of the disease since the filter insertion.