Observational study of galactic magnetic fields is limited by projected observables . Comparison with numerical simulations is helpful to understand the real structures , and observational visualization of numerical data is an important task . [ \citeauthoryear Machida et al.2018 ] have reported Faraday depth maps obtained from numerical simulations . They showed that the relation between azimuthal angle and Faraday depth depends on the inclination angle . In this paper , we investigate 100 MHz to 10 GHz radio synchrotron emission from spiral galaxies , using the data of global three-dimensional magneto-hydrodynamic simulations . We model internal and external Faraday depolarization at small scales and assume a frequency independent depolarization . It is found that the internal and external Faraday depolarization becomes comparable inside the disk and the dispersion of Faraday depth becomes about 4 { rad m ^ { -2 } } for face-on view and 40 { rad m ^ { -2 } } for edge-on view , respectively . The internal depolarization becomes ineffective in the halo . Because of the magnetic turbulence inside the disk , frequency independent depolarization works well and the polarization degree becomes 0.3 at high frequency . When the observed frequency is in the 100 MHz band , polarized intensity vanishes in the disk , while that from the halo can be observed . Because the remaining component of polarized intensity is weak in the halo and the polarization degree is about a few % , it may be difficult to observe that component . These results indicate that the structures of global magnetic fields in spiral galaxies could be elucidated , if broadband polarimetry such as that with the Square Kilometre Array is achieved .