We present herein the case of a 73-year-old Japanese man in whom a minute malignant islet cell tumor, 7 mm in maximal diameter, was discovered. The patient was admitted to our hospital to undergo a gastrectomy for gastric cancer, at which time preoperative ultrasonography (US) revealed a minute hypoechoic mass located in the head of the pancreas. Endoscopic retrograde pancreatography, angiography, computed tomography, and magnetic resonance imaging failed to reveal the features or location of the tumor; nowever, these details were obtained by endoscopic and intraoperative US. Although intraoperative fine-needle aspiration cytology of the tumor enabled a diagnosis of islet cell tumor to be made, it failed to provide enough material to evaluate the grade of malignancy, which was confirmed by histologic examination of the enucleated tumor, allowing the appropriate surgical procedure to be decided.