A treatment instrument includes a sheath that has a longitudinal axis, and a wire that has a loop portion which has a loop shape in a distal end, which is inserted into the sheath so as to be advanced and retracted, and which has a first bending portion bent to protrude toward an outside of the loop shape, a pair of second bending portions, and a pair of third bending portions, in an order from the distal end toward a proximal end. In the loop portion, a length of a second region from the pair of second bending portions to the pair of third bending portions is longer than a length of a first region from the first bending portion to the pair of second bending portions and a length of a third region from the pair of third bending portions to the proximal end of the loop portion. An angle formed inside the loop portion between the first region and the second region is equal to or smaller than an angle formed on the distal end side between the first region and the longitudinal axis.
