PROBLEM TO BE SOLVED: To provide a medical treatment instrument which quickly becomes firm in performing a treatment even if a soft sheath is used and keeps the bent state firmly.SOLUTION: In a medical treatment instrument which includes: a flexible sheath 2 that has a dog leg shaped bent part 6 in the vicinity of a distal end and is made of a close wound coil a treatment part 4 provided at a distal end of the sheath 2 an operation wire 15 which is inserted in the sheath 2 and is connected to the treatment part 4 and an operational part which is connected to a proximal end of the operation wire 15 and a proximal end of the sheath 2 and operates the treatment part 4 by operating to advance and retract the operation wire 15, a tension wire 7 for pulling the distal end of the sheath 2 to the proximal end side is disposed in a direction toward a bending inner side 6b of the bent part 6 rather than in an axial center of the sheath 2.
