A medical instrument includes a swingable operating section formed of a pair of forceps which rock around a first rocking axis, a tubular sheath having a distal end portion situated on the proximal end side of the operating section, the distal end portion having a circular-section portion having a circular cross section perpendicular to the longitudinal central axis thereof and a pair of flat portions formed by cutting the opposite sides of the circular-section portion and in sliding contact with the respective proximal end portions of the forceps, a manipulator which advances and retreats in a longitudinal direction of the sheath, thereby rocking the forceps around the first rocking axis, and a junction which connects the manipulator for rocking motion around a second rocking axis with respect to the forceps in the flat portions, the junction being situated on or near a reference plane passing through the longitudinal central axis of the sheath and extending parallel to the second rocking axis when the operating section is closed.