A belay descender device for a rope comprising a cam mounted rotating on a fixed flange to perform locking of the rope when said rope is under tension, and a lever articulated on the cam to perform progressive unlocking of the rope with a geared-down effect at the beginning of unlocking travel of the lever. The operating lever comprises transmission means collaborating with a guide ramp of the flange to interrupt the mechanical link with the cam after an intermediate position of the lever has been passed resulting in freeing from said ramp outside the gearing-down area. The end of the guide ramp of the flange corresponds to the anti-panic position of the transmission means, enabling automatic locking of the cam as soon as the gearing-down area has been passed.