In 2010, a 39-year-old woman presented with a cystic lesion, 16 mm in diameter, in the tail of the pancreas. Regular follow-ups were conducted to monitor this lesion; its diameter was found to increase to 45 mm in 2013. Thus, the patient was admitted to our hospital for further examination and treatment. Abdominal US, abdominal contrast-enhanced CT, and MRI showed a cystic lesion of 45 mm in diameter in the tail of the pancreas, which had internal septae and mural nodules inside. EUS revealed a cyst-in-cyst-like structure, with a thickened cystic wall along the entire circumference. Thus, distal pancreatectomy and splenectomy were performed on the basis of a diagnosis of mucinous cystic neoplasm. Histopathological examination of a resected specimen showed that the lesion comprised a substantial component of red-brown tone, with adjacent cystic components. The final diagnosis was an epidermoid cyst in an intrapancreatic accessory spleen.