  Active M-type stars are known to often produce superflares on the surface.
Radiation from stellar (super-)flares is important for the exoplanet
habitability, but the mechanisms are not well understood. In this paper, we
report simultaneous optical spectroscopic and photometric observations of a
stellar superflare on an active M dwarf YZ CMi with the 3.8-m Seimei telescope
and the $Transiting\, Exoplanet\, Survey\, Satellite$. The flare bolometric
energy was $1.3^{+1.6}_{-0.6} \times 10^{34} \,\rm{erg}$ and H$\alpha$ energy
was $3.0^{+0.1}_{-0.1} \times 10^{32} \,\rm{erg}$. The H$\alpha$ emission line
profile showed red asymmetry throughout the flare with a duration of $4.6-5.1
\,\rm{hrs}$. The velocity of the red asymmetry was $\sim 200-500
\,\rm{km\,s^{-1}}$ and line width of H$\alpha$ was broadened up to $34 \pm 14$
$\r{A}$. The redshifted velocity and line width of H$\alpha$ line decayed more
rapidly than the equivalent width, and their time evolutions are correlated
with that of the white-light emission. This indicates a possibility that the
white light, H$\alpha$ red asymmetry, and H$\alpha$ line broadening originate
from nearly the same site, i.e., the dense chromospheric condensation region
heated by non-thermal electrons. On the other hand, the flux ratio of the
redshifted excess components to the central components is enhanced one hour
after the flare onset. This may be due to the change of the main source of the
red asymmetry to the post-flare loops in the later phase of the flare.
