The invention concerns an apparatus for inspecting a surface, in particular the surface of a mill roll, comprising a housing wherein are arranged means for transmitting and receiving a light beam, and an optical coupling device arranged on the outer surface of a front wall of the housing. The optical coupling device comprises: (a) a component (5), called fluid dispenser, (b) a hollow cylindrical nozzle (11), and (c) a mobile tubular component (16), successively traversed by the light beam. The mobile tubular component (18) is arranged partly inside the cylindrical nozzle (11), so as to be able to slide inside said cylindrical nozzle, and partly exits therefrom in the extension thereof, towards the surface to be inspected (2). Different cross-sections (shoulders 14, 19) are provided in the cylindrical nozzle (11) and in the mobile tubular component (16), in particular for limiting said sliding movement. The mobile tubular component (16) is positioned in the cylindrical nozzle by the sole effect of the coupling fluid pressure on its end surfaces. The device can further comprise means (21, 22) for preventing the rotation of the tubular component (16) about its longitudinal axis.