A 42-year-old man with a history of suspected of Behcet's disease underwent oral steroid treatment. During follow-up, chest X-ray revealed an abnormal shadow of the mediastinum. Chest computed tomography(CT) showed a circumferential tumor around the descending thoracic aorta. Enhanced CT showed a lowly and uniformly enhanced tumor at delay phase. A mediastinal tumor was suspected, which prompted a biopsy of the periaortic tumor by video-assisted thoracic surgery (VATS). Histopathological diagnosis showed numerous immunogloblin G4 (IgG4)-positive plasma cells suggesting the possibility of IgG4-related periaortitis. However, based on the diagnostic criteria, the case was comprehensively diagnosed as probable IgG4-related periaortitis, steroid treatment may have affected blood IgG4-positive cells and tissues.