A medical instrument holding device includes a grasping portion onto which a medical instrument is mounted, an acceptant arm supported by a link mechanism, and a ball joint element adapted to be fixed to and released from the acceptant arm. The grasping portion is connected to the acceptant arm by the ball joint element arranged above or below the longitudinal axis of the acceptant arm. A total weight of the grasping portion is always balanced so as to apply to the acceptant arm (ball joint element) so that an operator can move the grasping portion under the condition that no total weight is applied to the operator.