The present invention provides an endoscope having an opening of a channel tube retracted from a distal end face having an observation window of the insert therein toward a proximal end side. When a plurality of polyps are removed, a picked-up polyp is moved back from the distal end face of the insert toward the proximal side of the insert in order for the picked-up polyp not to interfere with observation by the endoscope and operation by a cutting tool extending through the endoscope channel.