(a) The endoscope was inserted into the duodenum. m: mucosa, sm: submucosa, mp: muscularis propria, s: serosa. (b) After confirming the known tumor, slightly warm distilled water was injected into the duodenal lumen from the accessory channel, and the tumor was completely submerged. (c) The tumor was observed in underwater immersion and we observed until the lesion was slightly bulging from the mucosal surface (allow). During observation, the muscular layer was kept flat in a ring shape (allow head). (d) The snare was opened during underwater immersion and was subsequently pressed against the lesion site to confirm that the entire lesion entered the snare, and the lesion was strangulated. Resnaring was performed to confirm that the duodenal muscularis propria was not gripped, and the lesion was excised using an incision wave.