The case is 61 years old, female. The mediastinal mass was pointed out at a medical examination, and she visited our hospital. A plain computed tomography (CT) scan showed a 12×35 mm mass in the anterior mediastinum, and a thymoma was suspected. Magnetic resonance imaging( MRI) showed low signal at T1-weighted image and high signal at T2-weighted image. A benign cystic mass such as a thymic cyst and a pericardial cyst was suspected, and thoracoscopic resection was performed. Pathologically, a bronchogenic cyst was diagnosed.