An ascender/descender appliance for ascending and descending on a rope, comprising a body having attachment means to be connected to the user's harness and a clamping device able to be released on load. The clamping device is arranged outside the body and comprises a chain of metal links constituted by a succession of friction elements articulated on one another by connecting rods to form a series of gaps for the rope to pass in zigzag manner. Means are associated with the top link to trigger clamping of the ascender device.