  Faraday tomography is thought to be a powerful tool to explore cosmic
magnetic field. Broadband radio polarimetric data is essential to ensure the
quality of Faraday tomography, but such data is not easy to obtain because of
radio frequency interferences (RFIs). In this paper, we investigate optimum
frequency coverage of Faraday tomography so as to explore Faraday rotation
measure (RM) due to the intergalactic magnetic field (IGMF) in filaments of
galaxies. We adopt a simple model of the IGMF and estimate confidence intervals
of the model parameters using the Fisher information matrix. We find that
meaningful constraints for RM due to the IGMF are available with data at
multiple narrowbands which are scattered over the ultra-high frequency (UHF,
300 MHz - 3000 MHz). The optimum frequency depends on the Faraday thickness of
the Milky Way foreground. These results are obtained for a wide brightness
range of the background source including fast radio bursts (FRBs). We discuss
the relation between the polarized-intensity spectrum and the optimum
frequency.
