  We report on the spectroscopic confirmation of a massive quiescent galaxy at
$z_\mathrm{spec}=4.53$ in the COSMOS field. The object was first identified as
a galaxy with suppressed star formation at $z_\mathrm{phot}\sim4.65$ from the
COSMOS2020 catalog. The follow-up spectroscopy with Keck/MOSFIRE in the
$K$-band reveals faint [OII] emission and the Balmer break, indicative of
evolved stellar populations. We perform the spectral energy distribution
fitting using photometry and spectrum to infer physical properties. The
obtained stellar mass is high ($M_*\sim 10^{10.8}\,M_\odot$) and the current
star formation rate is more than 1 dex below that of main-sequence galaxies at
$z=4.5$. Its star formation history suggests that this galaxy experienced rapid
quenching from $z\sim 5$. The galaxy is among the youngest quiescent galaxies
confirmed so far at $z_\mathrm{spec}>3$ with $z_\mathrm{form}\sim5.2$
($200\,\mathrm{Myr}$ ago), which is the epoch when 50\% of total stellar mass
was formed. A unique aspect of the galaxy is that it is in an extremely dense
region; there are four massive star-forming galaxies at
$4.4<z_\mathrm{phot}<4.7$ located within 150 physical kpc from the galaxy.
Interestingly, three of them have strongly overlapping virial radii with that
of the central quiescent galaxy ($\sim 70\,\mathrm{kpc}$), suggesting that the
over-density region is likely the highest redshift candidate of a dense group
with a spectroscopically confirmed quiescent galaxy at the center. The group
provides us with a unique opportunity to gain insights into the role of the
group environment for quenching at $z\sim5$, which corresponds to the formation
epoch of massive elliptical galaxies in the local Universe.
