A system comprising an elongated medical member and an operation treatment instrument, the system comprising: an insertion portion having a long length, which is provided with a lumen through which an operation treatment instrument can be penetrated, the operation treatment instrument being adapted to be formed into an arcuate shape by a distal end thereof being pulled, and including a wire cutter for cutting a living body tissue, the lumen being provided along a longitudinal axis of the insertion portion, wherein the insertion portion is bendable and includes a distal end area including an opening portion of the lumen at a distal end surface of the insertion portion and a proximal end area having a predetermined length toward a proximal end side in an axis direction from the distal end area; the elongated medical member further comprises a guide portion adapted for guiding the operation treatment instrument extended from the lumen into a body cavity along the guide portion together with the insertion portion, wherein a proximal end is connected to a peripheral portion of the opening portion of the lumen and the guide portion is provided to extend from the proximal end to a distal end toward a direction in which the opening portion is opened, in order to guide a distal end part of the operation treatment instrument protruded from the opening portion of the lumen to a target area, and a slit having a predetermined length and formed from a distal end of a peripheral portion of the opening portion in a most proximal area of the opening portion, the slit including at a proximal end part thereof a long hole and being adapted for allowing the wire cutter to pass through from inside the lumen without an entirety of the wire cutter being guided out from the opening portion. an insertion portion having a long length, which is provided with a lumen through which an operation treatment instrument can be penetrated, the operation treatment instrument being adapted to be formed into an arcuate shape by a distal end thereof being pulled, and including a wire cutter for cutting a living body tissue, the lumen being provided along a longitudinal axis of the insertion portion, wherein the insertion portion is bendable and includes a distal end area including an opening portion of the lumen at a distal end surface of the insertion portion and a proximal end area having a predetermined length toward a proximal end side in an axis direction from the distal end area; the elongated medical member further comprises a guide portion adapted for guiding the operation treatment instrument extended from the lumen into a body cavity along the guide portion together with the insertion portion, wherein a proximal end is connected to a peripheral portion of the opening portion of the lumen and the guide portion is provided to extend from the proximal end to a distal end toward a direction in which the opening portion is opened, in order to guide a distal end part of the operation treatment instrument protruded from the opening portion of the lumen to a target area, and a slit having a predetermined length and formed from a distal end of a peripheral portion of the opening portion in a most proximal area of the opening portion, the slit including at a proximal end part thereof a long hole and being adapted for allowing the wire cutter to pass through from inside the lumen without an entirety of the wire cutter being guided out from the opening portion.