  We consider the possibility that cosmic magnetic field, instead of being
uniformly distributed, is strongly correlated with the large scale structure of
the universe. Then, the observed rotational measure of extra-galactic radio
sources would be caused mostly by the clumpy magnetic field in cosmological
filaments/sheets rather than by a uniform magnetic field, which was often
assumed in previous studies. As a model for the inhomogeneity of the
cosmological magnetic field, we adopt a cosmological hydrodynamic simulation,
where the field is passively included, and can approximately represent the real
field distribution with an arbitrary normalization for the field strength.
Then, we derive an upper limit of the magnetic field strength by comparing the
observed limit of rotational measure with the rotational measure expected from
the magnetic field geometry in the simulated model universe. The resulting
upper limit to the magnetic field in filaments and sheets is ${\bar B}_{fs} \la
1 \mu G$ which is $\sim10^3$ times higher than the previously quoted values.
This value is close to, but larger than, the equipartition magnetic field
strength in filaments and sheets. The amplification mechanism of the magnetic
field to the above strength is uncertain. The implications of such a strength
of the cosmic magnetic field are discussed.
