A 57-year-old woman was discovered to have a cystic tumor, 8 cm in diameter, at the pancreas tail, during routine screening with ultrasonography. The patient did not complain of tenderness, and no abdominal mass was palpable at physical examination. Enhanced computed tomography(CT) revealed that the tumor had mural nodules in the cyst wall, and we suspected it to be a malignant tumor that had occurred in the mucinous cystic neoplasm(MCN). Therefore, surgical resection was attempted, upon which the tumor was found to be hard and the surrounding tissue adhered widely to the stomach. We separated it carefully from the stomach and then performed a distal pancreatectomy. The cut surface revealed that the posterior wall of the cystic tumor was partly thickened, and microscopic examination revealed it to be invasive ductal carcinoma. No ovarian-like stroma was involved and some degree of dysplasia(PanIN 1-3) was found in the neighboring tissues. Therefore, we re-diagnosed it to be invasive ductal carcinoma of the pancreas derived from intraductal papillary mucinous tumor(IPMT), not from MCN. The patient received adjuvant chemotherapy, although 5 months later multiple lung metastases had appeared. The international consensus guidelines for management of IPMN and MCN of the pancreas suggest that they can usually be distinguished preoperatively, if there is a complete understanding of their clinical and imaging features. However, we sometimes find it difficult to distinguish the 2, because some IPMN or MCN cases have shared preoperative features. Herein, we report the case of invasive ductal carcinoma of the pancreas derived from IPMT that was originally misdiagnosed as a MCN.