We experienced a case of serous cystadenoma of the pancreas which was diagnosed after operation with a preoperative diagnosis of non-functioning islet cell tumor of the pancreas. A 68-year-old woman was pointed out having a tumor about 1cm in size at the pancreatic body on an abdominal ultrasonography when she had a physical examination. Ambulation began for close examination. Contrast enhanced CT revealed a dense tumor shadow. With magnetic resonance imaging, it showed low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. No accumulation of a contrast material revealed on Gd. ERP disclosed no abnormal pancreatic findings. EUS visualized a hypoechoic shadow. After admission, an angiography revealed accumulation of a contrast material. There were no abnormalities in blood biochemistry and endocrinological data. With a diagnosis of non-functioning islet cell tumor, a resection of the body and tail of the pancreas was performed on March 23, 1998. After the operation, serous cystadenoma was diagnosed histopathologically. With a recent progression of imaging diagnoses and a spread of medical checkup, cystadenomas of the pancreas have been increasingly detected. We must entertain serous cystadenoma of the pancreas as a probable differential diagnosis for pancreatic tumors.