A positioning device for positioning an aperture plate in an ion beam of an ion implantation system has two fixture parts that can be moved relative to each other by means of at least one positioning drive, of which the one fixture part can be connected or is connected to an abutment point that is disposed in a fixed location relative to an ion beam source, and the other fixture part can be connected or is connected to the aperture plate. An adjustment device has a fixture device and a display device. By means of the position-changing device, it is possible to change the position of at least one of the fixture parts relative to the positioning drive. By means of the display device it is possible to check whether the fixture parts are in a predetermined position relative to each other. On the first fixture part or in a part that is connected thereto in a fixed position, the display device has at least one reference mark and on the second fixture part at least one optical projection device by which means at least one optical marking that is assigned to a reference mark, of which at least one must be present, can be projected onto the first fixture part.