A magnetic drum separator (2), with a drum (6) rotatable about a rotational axis (4), a magnet arrangement (10) of a plurality of magnets (12) arranged in the interior (8) of the drum (6), a separation zone (18) in the exterior space (14) of the drum (6); a feed material (22) flows through the separation zone (18) and is there separable with the aid of a magnetic field (26) generated by the magnet arrangement (10), into a waste stream (30) and a recyclable material stream (28). A relative position (R) of at least one of the magnets (12) relative to the rotational axis (4) can be varied. A nominal magnitude (S) for a process value (78) on the drum separator (2) that is influenced by the separation behavior (32) is specified. At least one measurement device (74) detects an actual magnitude (I) of the process value (78), and a controller (82), which changes the relative position (R) of the at least one of the magnets (12), whereby the actual magnitude (I) is controlled to approach the nominal magnitude (S).