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