A 60-year-old woman was admitted to our hospital for dyspnea. Contrast-enhanced computed tomography( CT) revealed acute pulmonary artery thromboembolism. An echocardiogram showed a movable structure in the right atrium. Emergency surgery was performed under general anesthesia. The movable structure in the right atrium looked like a Chiari network with a diameter of over 5 cm, and no obvious thrombus was found. The patient had a good postoperative course and was discharged home on postoperative day 17. The Chiari network is a remnant structure of the fetal venous sinus valve, and its diagnosis was important for determining the appropriate treatment plan for this case.