  SN 2018ivc is an unusual type II supernova (SN II). It is a variant of SNe
IIL, which might represent a transitional case between SNe IIP with a massive
H-rich envelope, and IIb with only a small amount of the H-rich envelope.
However, SN 2018ivc shows an optical light curve evolution more complicated
than canonical SNe IIL. In this paper, we present the results of prompt
follow-up observations of SN 2018ivc with the Atacama Large
Millimeter/submillimeter Array (ALMA). Its synchrotron emission is similar to
that of SN IIb 1993J, suggesting that it is intrinsically an SN IIb-like
explosion of a He star with a modest (~0.5 - 1 Msun) extended H-rich envelope.
Its radio, optical, and X-ray light curves are explained primarily by the
interaction between the SN ejecta and the circumstellar material (CSM); we thus
suggest that it is a rare example (and the first involving the `canonical' SN
IIb ejecta) for which the multi-wavelength emission is powered mainly by the
SN-CSM interaction. The inner CSM density, reflecting the progenitor activity
in the final decade, is comparable to that of SN IIb 2013cu that showed a flash
spectral feature. The outer CSM density, and therefore the mass-loss rate in
the final ~200 years, is larger than that of SN 1993J by a factor of ~5. We
suggest that SN 2018ivc represents a missing link between SNe IIP and IIb/Ib/Ic
in the binary evolution scenario.
