  We have investigated the directional dichroism of magnetic resonance spectra
in the polar ferromagnet GaV$_4$S$_8$. While four types of structural domains
are energetically degenerated under zero field, the magnetic resonance for each
domain is well separated by applying magnetic fields due to uniaxial magnetic
anisotropy. Consequently, the directional dichroism as large as 20 % is clearly
observed without domain cancellation. The present observation therefore
demonstrates that not only magnetoelectric mono-domain crystals but also
magnetoelectric multi-domain specimens can be used to realize microwave
(optical) diodes owing to the lack of inversion domains.
