  We present the first radio polarimetric observations of a fast-rising blue
optical transient, AT2018cow. Two epochs of polarimetry with additional
coincident photometry were performed with the Atacama Large
Millimeter/submillimeter Array (ALMA). The overall photometric results based on
simultaneous observations in the 100 and 230 GHz bands are consistent with the
non-thermal radiation model reported by Ho et al. (2019) and indicate that the
spectral peaks ($\sim110$ GHz at the first epoch and $\sim67$ GHz at the second
epoch) represent the synchrotron self-absorption frequency. The non-detection
of linear polarization with $<$0.15% in the 230 GHz band at the phase when the
effect of synchrotron self-absorption was quite small in the band may be
explained by internal Faraday depolarization with high circumburst density and
strong magnetic field. This result supports the stellar explosion scenario
rather than the tidal disruption model. The maximum energy of accelerating
particles at the shocks of AT2018cow-like objects is also discussed.
