  We present 850 $\mu$m polarimetric observations toward the Serpens Main
molecular cloud obtained using the POL-2 polarimeter on the James Clerk Maxwell
Telescope (JCMT) as part of the B-fields In STar-forming Region Observations
(BISTRO) survey. These observations probe the magnetic field morphology of the
Serpens Main molecular cloud on about 6000 au scales, which consists of cores
and six filaments with different physical properties such as density and star
formation activity. Using the histogram of relative orientation (HRO)
technique, we find that magnetic fields are parallel to filaments in less dense
filamentary structures where $N_{H_2} < 0.93\times 10^{22}$ cm$^{-2}$ (magnetic
fields perpendicular to density gradients), while being perpendicular to
filaments (magnetic fields parallel to density gradients) in dense filamentary
structures with star formation activity. Moreover, applying the HRO technique
to denser core regions, we find that magnetic field orientations change to
become perpendicular to density gradients again at $N_{H_2} \approx 4.6 \times
10^{22}$ cm$^{-2}$. This can be interpreted as a signature of core formation.
At $N_{H_2} \approx 16 \times 10^{22}$ cm$^{-2}$ magnetic fields change back to
being parallel to density gradients once again, which can be understood to be
due to magnetic fields being dragged in by infalling material. In addition, we
estimate the magnetic field strengths of the filaments ($B_{POS} =
60-300~\mu$G)) using the Davis-Chandrasekhar-Fermi method and discuss whether
the filaments are gravitationally unstable based on magnetic field and
turbulence energy densities.
