To provide a permanent magnet demagnetization device for erasing storage of a magnetic data storage device.

The permanent magnet demagnetization device includes magnetization segments (21 to 30) in which one or a plurality of pairs of magnet groups (14 and 16) are arranged in the shape of a Halbach array. The magnet groups (14 and 16) demarcate a gap (18) through which a magnetic data storage medium (12) is made to pass in a direction (20) for going across the segments (21 to 30). The magnetization lateral surfaces (36) of the magnet groups (14 and 16) confront each other, which generates a strong magnetic field that demagnetizes the magnetic data storage medium (12) made to pass through the gap (18).