  Magnetic fields play essential roles in various astronomical objects. Radio
astronomy has revealed that magnetic fields are ubiquitous in our Universe.
However, the real origin and evolution of magnetic fields is poorly proven. In
order to advance our knowledge of cosmic magnetism in coming decades, the
Square Kilometre Array (SKA) should have supreme sensitivity than ever before,
which provides numerous observation points in the cosmic space. Furthermore,
the SKA should be designed to facilitate wideband polarimetry so as to allow us
to examine sightline structures of magnetic fields by means of depolarization
and Faraday Tomography. The SKA will be able to drive cosmic magnetism of the
interstellar medium, the Milky Way, galaxies, AGN, galaxy clusters, and
potentially the cosmic web which may preserve information of the primeval
Universe. The Japan SKA Consortium (SKA-JP) Magnetism Science Working Group
(SWG) proposes the project "Resolving 4-D Nature of Magnetism with
Depolarization and Faraday Tomography", which contains ten scientific use
cases.
