Abstract: An apparatus for holding a medical instrument, such as an endoscope, in place without exerting an undue force on an insertion hole in an abdominal cavity of a subject even when the arm of a scope-holding section is unlocked, includes a four-link parallelogram mechanism. The four-link parallelogram mechanism includes a support arm for supporting a treating tool, an arm section parallel to the support arm and upper and lower arm sections connected to the upper and lower end sides of the support arm and arm section in a parallel relation. A support mechanism supports the arm sections of the four-link parallelogram mechanism to allow the arm sections to be moved in left/right and up/down directions, while being rotated, so that a straight line connecting an intersecting point P of a center line of the treating instrument inserted into an insertion hole in an abdominal wall of a subject and the center of the insertion hole to spherical bearings of the arm is set parallel to the upper and lower arm sections.