  Type IIn/Ia-CSM supernovae (SNe IIn/Ia-CSM) are classified by their
characteristic spectra, which exhibit narrow hydrogen emission lines
originating from a strong interaction with a circumstellar medium (CSM)
together with broad lines of intermediate-mass elements. We performed intensive
follow-up observations of SN IIn/Ia-CSM 2020uem, including photometry,
spectroscopy, and polarimetry. In this paper, we focus on the results of
polarimetry. We performed imaging polarimetry at $66$ days and
spectropolarimetry at $103$ days after the discovery. SN 2020uem shows a high
continuum polarization of $1.0-1.5\%$ without wavelength dependence. Besides,
the polarization degree and position angle keep roughly constant. These results
suggest that SN 2020uem is powered by a strong interaction with a confined and
aspherical CSM. We performed a simple polarization modeling, based on which we
suggest that SN 2020uem has an equatorial-disk/torus CSM. Besides, we performed
semi-analytic light-curve modeling and estimated the CSM mass. We revealed that
the mass-loss rate in the final few hundred years immediately before the
explosion of SN 2020uem is in the range of $0.01 - 0.05 {\rm
~M_{\odot}~yr^{-1}}$, and that the total CSM mass is $0.5-4 {\rm ~M_{\odot}}$.
The CSM mass can be accommodated by not only a red supergiant (RSG) but a red
giant (RG) or an asymptotic-giant-branch (AGB) star. As a possible progenitor
scenario of SN 2020uem, we propose a white-dwarf binary system including an RG,
RSG or AGB star, especially a merger scenario via common envelope evolution,
i.e., the core-degenerate scenario or its variant.
