  Solar flares are often accompanied by filament/prominence eruptions
($\sim10^{4}$ K and $\sim 10^{10-11}$ cm$^{-3}$), sometimes leading to coronal
mass ejections (CMEs) that directly affect the Earth's environment.
`Superflares' are found on some active solar-type (G-type main-sequence) stars,
but the association of filament eruptions/CMEs has not been established. Here
we show that our optical spectroscopic observation of the young solar-type star
EK Draconis reveals the evidence for a stellar filament eruption associated
with a superflare. This superflare emitted a radiated energy of
$2.0\times10^{33}$ erg, and blue-shifted hydrogen absorption component with a
large velocity of $-510$ km s$^{-1}$ was observed shortly after. The temporal
changes in the spectra greatly resemble those of solar filament eruptions.
Comparing this eruption with solar filament eruptions in terms of the length
scale and velocity strongly suggests that a stellar CME occurred. The erupted
filament mass of $1.1\times10^{18}$ g is 10 times larger than those of the
largest solar CMEs. The massive filament eruption and an associated CME provide
the opportunity to evaluate how they affect the environment of young
exoplanets/young Earth and stellar mass/angular-momentum evolution.
