The present disclosure provides a magnet device for providing the magnetic density changes/magnetic field changes in relation to a detecting position. The magnet device comprises a magnet member adapted to be mounted on a rotatable shaft and be rotated for producing the magnetic density changes/magnetic field changes in relation to the detecting position when the magnet member is rotating around the rotatable shaft. To concentrate/condense the density of the magnetic density changes/magnetic field changes, the magnet device includes a magnetic flux density concentrator.