an adjustment device for a rotational adjustment of two centrically mounted adapter pieces especially for a seat , preferably for a seat of a motor vehicle , includes a planetary gear with a sun wheel and planetary wheels . the planetary gear has a first housing part and a second housing part each with an internal toothing . the housing parts can be rotated relative to one another and are captively held on one another . both housing parts have an edge flange . a separate retaining ring , which overlaps the edge flanges , holds the housing parts together . this allows to assemble the adjustment device in a simple manner and at low cost and ensures that the adjustment device functions even when subjected to high stress .