  External galaxies often intervene in front of background radio sources such
as quasars and radio galaxies. Linear polarization of the background emission
is depolarized by Faraday rotation of inhomogeneous magnetized plasma of the
intervening galaxies. Exploring the depolarizing intervening galaxies (DINGs)
can be a powerful tool to investigate the cosmological evolution of the
galactic magnetic field. In this paper, we investigate the effects of DINGs on
background radio emission using theoretical DING models. We find that complex
structures of galaxy result in complicated depolarization features and the
Faraday dispersion functions (FDFs), but, for the features of depolarizations
and FDFs, the global component of magnetic fields is important. We show the
simplest results with ring magnetic field in the galactic disk. We find that
the degree of depolarization significantly depends on the inclination angle and
the impact parameter of the DING. We found that the larger the standard
deviation, the more likely it is that depolarization will occur. The FDF
represents the RM structure within the beam. The FDF exhibits multi-components
due mainly to the RM structure within the beam and the fraction of the DING
that covers the background emission (the filling factor). The peak Faraday
depth of the FDF is different from the beam-averaged RM of the DING. The
Monte-Carlo simulations indicate that DING's contribution to the standard
deviation of observed RMs follows $\sigma_{\rm RM} \propto 1/{(1+z)^k}$ with $k
\sim 2.7$ and exhibits a steeper redshift dependence than the wavelength
squared. DINGs will have a significant impact on RM catalogs created by future
survey projects such as the SKA and SKA Precursor/Pathfinder.
