The gripping mechanism (3) according to the present invention includes a first gripping piece (5) and a second gripping piece (6) connected to each other around a swing axis (A1) and a swing axis ( A pulling pulley (7) supported by the second gripping piece (6) so as to be rotatable about a rotation axis (A2) parallel to A1), and a tip is fixed to one gripping piece (5). The resultant force (F ′) of the tension (f1 ′, f2 ′) on both sides of the traction pulley (7) across A2) is the rotational moment in the closing direction of the second gripping piece (6) around the swing axis (A1). A wire (8) wound around the pulling pulley (7) so as to generate (M ′).本発明の把持機構（３）は、揺動軸線（Ａ１）回りに相互に揺動可能に連結された第１の把持片（５）および第２の把持片（６）と、揺動軸線（Ａ１）に平行な回転軸線（Ａ２）回りに回転可能に第２の把持片（６）に支持された牽引プーリ（７）と、先端が一方の把持片（５）に固定され、回転軸線（Ａ２）を挟んだ牽引プーリ（７）の両側における張力（ｆ１’，ｆ２’）の合力（Ｆ’）が揺動軸線（Ａ１）回りの第２の把持片（６）の閉方向の回転モーメント（Ｍ’）を発生するように牽引プーリ（７）に巻き掛けられたワイヤ（８）とを備える。
