  Long-term observations of synchrotron emission from supernovae (SNe),
covering more than a year after the explosion, provide a unique opportunity to
study the poorly-understood evolution of massive stars in the final millennium
of their lives via changes in the mass-loss rate. Here, we present a result of
our long-term monitoring of a peculiar type IIL SN 2018ivc, using the Atacama
Large Millimeter/submillimeter Array (ALMA). Following the initial decay, it
showed unprecedented rebrightening starting at ~ a year after the explosion.
This is one of the rare examples showing such rebrightening in the synchrotron
emission, and the first case at millimeter wavelengths. We find it to be in the
optically-thin regime unlike the optically-thick centimeter emission. As such,
we can robustly reconstruct the distribution of the circumstellar matter (CSM)
and thus the mass-loss history in the final ~1,000 years. We find that the
progenitor of SN 2018ivc had experienced a very high mass-loss rate >~10^{-3}
Msun/yr ~1,500 years before the explosion, which was followed by a moderately
high mass-loss rate (>~10^{-4} Msun/yr) up until the explosion. From this
behavior, we suggest SN 2018ivc represents an extreme version of a binary
evolution toward SNe IIb, which bridges the hydrogen-poor SNe (toward SNe Ib/c,
without a hydrogen envelope) and hydrogen-rich SNe (SNe IIP, with a massive
envelope).
