The diagnosis of intraductal papillary mucinous tumor (IPMT) can be performed under the direct visualization of peroral pancreatoscopy (PPS), although the visible field with PPS is limited and endoscopic accessories cannot be easily applied. PPS is useful in cases with IPMT not only for the main duct lesions but also for some of the branch lesions that can be shown through the dilated branch duct. PPS is useful for diagnosing IPMT because histologic diagnosis is possible from biopsy materials obtained by PPS or with radiograph guidance. Histologic findings can be suspected from the appearance and degree of the protrusion of the lesions in the cystic lesion or in the main duct. Thirty patients with IPMT were resected and confirmed histologically. Among them, 26 cases were examined by PPS. Detection rates of the 12 cases with polypoid tumor greater than 3 mm were 67% by PPS, 92% by endoscopic ultrasonography, and 100% by intraductal ultrasonography. Among the 6 cases of adenocarcinoma, 4 cases showed a tumor mass greater than 10 mm.