A medical instrument comprising: a tubular main body that is long and flexible; a plurality of operating lines which are inserted into the tubular main body, each of the operating lines having a tip end portion connected to a distal portion of the tubular main body; an operating part main body provided in a base end portion of the tubular main body; a bending operating part which is provided such that the bending operating part is configured to rotate relative to the operating part main body and to slide between a fixation position and an operation position, to which a base end portion of each operating line is fixed, and which bends the distal portion of the tubular main body by individually applying a pulling force to the plurality of operating lines via the rotation of the bending operating part; and a path defining part that defines a path of each operating line inside the bending operating part, wherein when the bending operating part is at the fixation position, the operating lines are in a relaxed state, and when the bending operating part is at the operation position, the operating lines are in a state that is more strained than the relaxed state, characterized in that the instrument further comprises a deviation preventive member that prevents each operating line from deviating from the path defined by the path defining part, wherein the deviation preventive member is provided with a second notched portion through which the operating lines pass, and a peripheral boundary edge of the second notched portion has a chamfered shape in which the peripheral boundary edge is rounded in a thickness direction of the deviation preventive member. a tubular main body that is long and flexible; a plurality of operating lines which are inserted into the tubular main body, each of the operating lines having a tip end portion connected to a distal portion of the tubular main body; an operating part main body provided in a base end portion of the tubular main body; a bending operating part which is provided such that the bending operating part is configured to rotate relative to the operating part main body and to slide between a fixation position and an operation position, to which a base end portion of each operating line is fixed, and which bends the distal portion of the tubular main body by individually applying a pulling force to the plurality of operating lines via the rotation of the bending operating part; and a path defining part that defines a path of each operating line inside the bending operating part, wherein when the bending operating part is at the fixation position, the operating lines are in a relaxed state, and when the bending operating part is at the operation position, the operating lines are in a state that is more strained than the relaxed state, characterized in that the instrument further comprises a deviation preventive member that prevents each operating line from deviating from the path defined by the path defining part, wherein the deviation preventive member is provided with a second notched portion through which the operating lines pass, and a peripheral boundary edge of the second notched portion has a chamfered shape in which the peripheral boundary edge is rounded in a thickness direction of the deviation preventive member.