A constriction removal method using a high-frequency knife (8) and a collecting instrument (7) having a longitudinal axis and an increasing-diameter part (12), whose diameter can be increased, at the distal end, the constriction removal method including: inserting the collecting instrument (7) from a proximal end opening of a constricted lumen of a constricted portion； allowing the increasing-diameter part (12) to pass through the constricted lumen and to project from the distal-end opening of the constricted lumen and allowing the diameter of the increasing-diameter part (12) having passed through the constricted lumen to increase toward the radially outer side of the constricted lumen； hooking ends of the increasing-diameter part (12) having passed through the constricted lumen on an edge of the distal-end opening of the constricted lumen； positioning the high-frequency knife (8) on the radially outer side of the proximal end opening of the constricted lumen； cylindrically coring out the constricted lumen with the high-frequency knife (8) in a state in which the ends of the increasing-diameter part (12) are hooked on the edge of the distal-end opening of the constricted lumen； and collecting the cored-out constricted lumen by pulling the collecting instrument (7) toward the proximal end of the collecting instrument (7).
