Abstract: To evaluate the condition of the bearing, particularly of bearings used in exhaust gas turbo-superchargers, the end cut surface of a light guide bundle having individual light guide fibers or filaments is exposed to a gap between rotating and stationary surfaces, for example the end portion of a sleeve bearing, or a disk attached to the rotating shaft portion of another type of bearing, the light being passed to the bearing gap being evaluated either by counting the light fibers which have a light intensity indicative of the size of the gap at any instant of time, or determining the average light output. If a disk is used, light can be transmitted from one group of bundles through a gap to another group of bundles, the size of the gap varying in dependence on the quality of centricity of the bearing and other operating conditions. Preferably, the bundle is enclosed within a tube, with clearance, and compressed air is supplied within the tube to clean the end surfaces.