  We report a one-second-cadence wide-field survey for M-dwarf flares using the
Tomo-e Gozen camera mounted on the Kiso Schmidt telescope. We detect 22 flares
from M3-M5 dwarfs with rise times and amplitudes ranging from $5\, \mathrm{sec}
\lesssim t_\mathrm{rise} \lesssim 100\,\mathrm{sec}$ and $0.5 \lesssim \Delta
F/F_{\star} \lesssim 20$, respectively. The flare light curves mostly show
steeper rises and shallower decays than those obtained from the Kepler
one-minute cadence data and tend to have flat peak structures. Assuming a
blackbody spectrum with temperatures of $9,000-15,000\,\mathrm{K}$, the peak
luminosities and bolometric energies are estimated to be
$10^{29}\,\mathrm{erg\,sec^{-1}} \lesssim L_\mathrm{peak} \lesssim
10^{31}\,\mathrm{erg\,sec^{-1}}$ and $10^{31}\,\mathrm{erg} \lesssim E_{\rm
bol} \lesssim 10^{34}\,\mathrm{erg}$, which constitutes the bright end of fast
optical flares for M dwarfs. We confirm that more than 90\% of the host stars
of the detected flares are magnetically active based on their H$\alpha$
emission line intensities obtained by LAMOST. The estimated occurrence rate of
the detected flares is $\sim 0.7$ per day per an active star, indicating they
are common in magnetically active M dwarfs. We argue that the flare light
curves can be explained by the chromospheric compression model; the rise time
is broadly consistent with the Alfv\'en transit time of a magnetic loop with a
length scale of $l_\mathrm{loop} \sim 10^4\,\mathrm{km}$ and a field strength
of $1,000\,\mathrm{G}$, while the decay time is likely determined by the
radiative cooling of the compressed chromosphere down to near the photosphere
with a temperature of $\gtrsim 10,000\,\mathrm{K}$. These flares from M dwarfs
could be a major contamination source for a future search of fast optical
transients of unknown types.
