We report a rare case of a giant celiac aneurysm complicated with nosocomial acute aortic dissection and idiopathic thrombocytonenic purpura (ITP). A 75-year-old man with ITP complained of abdominal swelling. Enhanced computed tomography (CT) showed a giant celiac aneurysm 72 mm in size. Surgery repair was scheduled and platelet count increased by intravenous administration of immunoglobulin. After admission, he complained of back pain. CT showed aortic dissection (DeBakey classification : IIIb) and a celiac aneurysm enlarged to 78 mm. He underwent surgical repair for a giant celiac aneurysm and splenectomy after management with medial therapy.