A medical instrument comprising: an insertion unit having flexibility, the insertion unit extending along a longitudinal axis; a treatment unit having a first grasping member (11; 81; 91; 96) provided at a distal end of the insertion unit and a second grasping member (12; 82; 92; 97) configured to approach and separate from the first grasping member (11; 81; 91; 96); a rotating body configured to be rotatably installed on a rotation shaft fixed to the first grasping member (11; 81; 91; 96); an open-close link having a first end rotatably connected to a first rotation shaft provided on the second grasping member (12;82; 92; 97) and a second end rotatably connected to a second rotation shaft provided on the rotating body at a distance from the rotation shaft, the second end being connected to the first end; a manipulating member configured to rotate the rotating body; and a manipulation unit provided at a proximal end of the manipulating member, characterized in that the medical instrument further comprises a parallel link (15, 16) having a first connecting site rotatably connected to the first grasping member (11; 81; 91; 96) and a second connecting site rotatably connected to the second grasping member (12; 82; 92; 97) and connected to the first connecting site, such that the first grasping member (11; 81; 91; 96) and the second grasping member (12; 82; 92; 97) are maintained to be substantially parallel to each other in a position where the first grasping member (11; 81; 91; 96) and the second grasping member (12; 82; 92; 97) approach each other and a position where the first grasping member (11; 81; 91; 96) and the second grasping member (12; 82; 92; 97) separate from each other, wherein, while the rotating body rotates about the rotation shaft by the manipulating member being pulled toward the manipulation unit by the manipulation unit and moves the second connecting site of the parallel link (15, 16) toward a proximal end of the treatment unit, the rotating body causes the first grasping member (11; 81; 91; 96) and the second grasping member (12; 82; 92; 97) to approach each other while the second rotation shaft approaches an imaginary straight line connecting axes of the rotation shaft and the first rotation shaft. an insertion unit having flexibility, the insertion unit extending along a longitudinal axis; a treatment unit having a first grasping member (11; 81; 91; 96) provided at a distal end of the insertion unit and a second grasping member (12; 82; 92; 97) configured to approach and separate from the first grasping member (11; 81; 91; 96); a rotating body configured to be rotatably installed on a rotation shaft fixed to the first grasping member (11; 81; 91; 96); an open-close link having a first end rotatably connected to a first rotation shaft provided on the second grasping member (12;82; 92; 97) and a second end rotatably connected to a second rotation shaft provided on the rotating body at a distance from the rotation shaft, the second end being connected to the first end; a manipulating member configured to rotate the rotating body; and a manipulation unit provided at a proximal end of the manipulating member, characterized in that the medical instrument further comprises a parallel link (15, 16) having a first connecting site rotatably connected to the first grasping member (11; 81; 91; 96) and a second connecting site rotatably connected to the second grasping member (12; 82; 92; 97) and connected to the first connecting site, such that the first grasping member (11; 81; 91; 96) and the second grasping member (12; 82; 92; 97) are maintained to be substantially parallel to each other in a position where the first grasping member (11; 81; 91; 96) and the second grasping member (12; 82; 92; 97) approach each other and a position where the first grasping member (11; 81; 91; 96) and the second grasping member (12; 82; 92; 97) separate from each other, wherein, while the rotating body rotates about the rotation shaft by the manipulating member being pulled toward the manipulation unit by the manipulation unit and moves the second connecting site of the parallel link (15, 16) toward a proximal end of the treatment unit, the rotating body causes the first grasping member (11; 81; 91; 96) and the second grasping member (12; 82; 92; 97) to approach each other while the second rotation shaft approaches an imaginary straight line connecting axes of the rotation shaft and the first rotation shaft.