A scanning unit including a light source for illuminating a scale by a light beam and a detector for detecting the light beam which has been modulated by the scale as a function of a relative position between two objects. An inner chamber of the scanning unit in which the light beam extends is closed off in a dust-proof manner against an outer chamber, and the inner chamber is closed off by a transparent body, through which the light beam passes. The inner chamber is connected with the outer chamber via a filter which is dust-proof, but permeable to gas and vapor.