A distal end cover (23) of a calculus fragmentation device (1) as an endoscopic treatment instrument has a first opening (23a) which is open toward a coil sheath (20), a second opening (23b) which is open toward an opposite side to the first opening (23a), a passage (24) configured to connect the first opening (23a) and the second opening (23b), an inner peripheral surface (25) connected to the first opening (23a), provided in a passage (24), and coming into contact with elastic wires (7), and a curved surface (26) bent to have a radius of curvature (R0) larger than a wall thickness (t) of the distal end cover (23) at a cross section including a center line (X1) of the distal end cover (23), provided in the passage (24) to connect the inner peripheral surface (25) and the second opening (23b), and allowing the elastic wires (7) to slide thereon.
