A magnetic marker comprises a magnetically switchable wire and a magnetic
casing that covers the magnetically switchable wire. The magnetically
switchable wire is formed of a magnetic material that undergoes occurrence
of sharp magnetic inversion when an alternating field of intensity higher
than its coercive force is applied to it. The magnetic casing is formed of
a magnetically hard or semihard magnetic material and can apply a bias
magnetic field to the magnetically switchable wire to prevent magnetic
inversion of the magnetically switchable wire. Heat-treated portions and
high-coercivity regions, which are not heat-treated, are formed
alternately in the longitudinal direction on the magnetic casing. The
heat-treated portions are given magnetic properties different from
magnetic properties essential to the magnetic casing by heat treatment
such as annealing.