Each pivotal shaft normally urged in one direction by the urging means drives the movable barrel to push the pressure spring, so that the clutch disposed either to the inner ball or to the outer ball in each joint is brought into press contact with the other ball to lock the joint. Accordingly, once the holding arm holds a tool, the holding force of the arm does not decline with time. Since the pressure applied to the pressure spring by the movable barrel is released by turning the pivotal shaft in the opposite direction by the driving means, each joint assumes an unlocked state. Thus, the tool held at the free end portion of the holding arm can be moved to a desired position, and also the holding position can be changed very easily.