  Young solar-type stars are known to show frequent "superflares", which may
severely influence the habitable worlds on young planets via intense radiations
and coronal mass ejections. Here we report an optical spectroscopic and
photometric observation of a long-duration superflare on the young solar-type
star EK Draconis (50-120 Myr age) with the Seimei telescope and $Transiting$
$Exoplanet$ $Survey$ $Satellite$ ($TESS$). The flare energy
2.6$\times$10$^{34}$ erg and white-light flare duration 2.2 hr are much larger
than those of the largest solar flares, and this is the largest superflare on a
solar-type star ever detected by optical spectroscopy. The H$\alpha$ emission
profile shows no significant line asymmetry, meaning no signature of a filament
eruption, unlike the only previous detection of a superflare on this star
(Namekata et al. 2021, $Nat.Astron$). Also, it did not show significant line
broadening, indicating that the non-thermal heating at the flare footpoints are
not essential or that the footpoints are behind the limb. The time evolution
and duration of the H$\alpha$ flare are surprisingly almost the same as those
of the white-light flare, which is different from general M-dwarf
(super-)flares and solar flares. This unexpected time evolution may suggest
that different radiation mechanisms than general solar flares are predominant,
as follows: (1) radiation from (off-limb) flare loops, and (2) re-radiation via
radiative backwarming, in both of which the cooling timescales of flare loops
could determine the timescales of H$\alpha$ and white light.
