jats:titleAbstract</jats:title> jats:pWe present 850 jats:italicμ</jats:italic>m polarimetric observations toward the Serpens Main molecular cloud obtained using the POL-2 polarimeter on the James Clerk Maxwell Telescope as part of the B-fields In STar-forming Region Observations 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 jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msub mml:mrow mml:miN</mml:mi> </mml:mrow> mml:mrow mml:msub mml:mrow <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> mml:mrow mml:mn2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> mml:mo<</mml:mo> mml:mn0.93</mml:mn> mml:mo×</mml:mo> mml:msup mml:mrow mml:mn10</mml:mn> </mml:mrow> mml:mrow mml:mn22</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac4bbeieqn1.gif" xlink:type="simple" /> </jats:inline-formula> cmjats:sup−2</jats:sup> (magnetic fields perpendicular to density gradients), while they are 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 jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msub mml:mrow mml:miN</mml:mi> </mml:mrow> mml:mrow mml:msub mml:mrow <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> mml:mrow mml:mn2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> mml:mo≈</mml:mo> mml:mn4.6</mml:mn> mml:mo×</mml:mo> mml:msup mml:mrow mml:mn10</mml:mn> </mml:mrow> mml:mrow mml:mn22</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac4bbeieqn2.gif" xlink:type="simple" /> </jats:inline-formula> cmjats:sup−2</jats:sup>. This can be interpreted as a signature of core formation. At jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msub mml:mrow mml:miN</mml:mi> </mml:mrow> mml:mrow mml:msub mml:mrow <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> mml:mrow mml:mn2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> mml:mo≈</mml:mo> mml:mn16</mml:mn> mml:mo×</mml:mo> mml:msup mml:mrow mml:mn10</mml:mn> </mml:mrow> mml:mrow mml:mn22</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac4bbeieqn3.gif" xlink:type="simple" /> </jats:inline-formula> cmjats:sup−2</jats:sup>, 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 (jats:italicB</jats:italic> jats:subPOS</jats:sub> = 60–300 jats:italicμ</jats:italic>G)) using the Davis–Chandrasekhar–Fermi method and discuss whether the filaments are gravitationally unstable based on magnetic field and turbulence energy densities.</jats:p>