A device for attaching on a spectacle lens a block for mounting on a lens edging or bevelling machine comprises fixed mounting block holding means, lens holding means located substantially in one plane and movable between a rest or centering position, in which the lens to be blocked is at a distance from the mounting block and a blocking position for securing the mounting block on the lens. This device further comprises centering marks located substantially in the plane formed by the lens holding means when no lens is placed on said holding means. The centering marks are movable in a direction substantially perpendicular to said plane in such a way that they are always in contact with the surface of a lens placed on its holding means, said surface being the surface adapted to receive subsequently the mounting block.