A metal detector (20) has a metallic enclosure (21) with entrance and exit apertures (30, 31). A coil system inside the enclosure has an energized transmitter coil. First and second receiver coils (24, 25) bound a detection zone (28) between the apertures, through which inspected objects (2) travel. The receiver coils are on opposite sides of the transmitter coil, with regard to the direction of travel (13). The receiver coils are connected in series, but their windings are wires oppositely in a rotational sense. Metal contamination in the object generates a detection signal in the receiver coils. A first and second flange (26), arranged at the entrance and exit, cancel the undesirable influence of metal contamination beyond a first distance upstream of and a second distance downstream of the coil system. The respective flanges differ from each other and, consequently, the first and second distances are different.