One treatment instrument includes the sheath with a longitudinal axis, an and conducting wire, it has a loop segments, it, which has, is inserted into sheath to be advanced and retracted by the loop shape in a distal end, it makes the first bending part bend towards the outside of loop shape, a pair of second bending part, and a pair of of third bending part are prominent, by a proximal end distally toward in an order. In loop segments, the length in one second area from a pair of second bending part to a pair of of third bending part is longer than from the first bending part to the length in one first area of a pair of second bending part and the length in the third area from a pair of of third bending part to the proximal end of loop segments. It is equal to or less than one between first area and longitudinal axis in the angle formed between first area and the secondth area in loop segments and is formed in angle on the side of end.
