  In this Letter we report a discovery of a prominent flash of a peculiar
overluminous Type Ia supernova, SN 2020hvf, in about 5 hours of the supernova
explosion by the first wide-field mosaic CMOS sensor imager, the Tomo-e Gozen
Camera. The fast evolution of the early flash was captured by intensive
intranight observations via the Tomo-e Gozen high-cadence survey. Numerical
simulations show that such a prominent and fast early emission is most likely
generated from an interaction between $0.01~M_{\odot}$ circumstellar material
(CSM) extending to a distance of $\sim$$10^{13}~\text{cm}$ and supernova ejecta
soon after the explosion, indicating a confined dense CSM formation at the
final evolution stage of the progenitor of SN 2020hvf. Based on the CSM-ejecta
interaction-induced early flash, the overluminous light curve, and the high
ejecta velocity of SN 2020hvf, we suggest that the SN 2020hvf may originate
from a thermonuclear explosion of a super-Chandrasekhar-mass white dwarf
("super-$M\rm_{Ch}$ WD"). Systematical investigations on explosion mechanisms
and hydrodynamic simulations of the super-$M\rm_{Ch}$ WD explosion are required
to further test the suggested scenario and understand the progenitor of this
peculiar supernova.
