  Stellar coronal mass ejections (CMEs) have recently received much attention
for their impacts on exoplanets and stellar evolution. Detecting prominence
eruptions, the initial phase of CMEs, as the blue-shifted excess component of
Balmer lines is a technique to capture stellar CMEs. However, most of
prominence eruptions identified thus far have been slow and less than the
surface escape velocity. Therefore, whether these eruptions were developing
into CMEs remained unknown. In this study, we conducted simultaneous optical
photometric observations with Transiting Exoplanet Survey Satellite and optical
spectroscopic observations with the 3.8m Seimei Telescope for the RS CVn-type
star V1355 Orionis that frequently produces large-scale superflares. We
detected a superflare releasing $7.0 \times 10^{35} \: \mathrm{erg}$. In the
early stage of this flare, a blue-shifted excess component of $\mathrm{H
\alpha}$ extending its velocity up to $760-1690 \: \mathrm{km \: s^{-1}}$ was
observed and thought to originate from prominence eruptions. The velocity
greatly exceeds the escape velocity (i.e., $\sim 350 \: \mathrm{km \:
s^{-1}}$), which provides important evidence that stellar prominence eruptions
can develop into CMEs. Furthermore, we found that the prominence is very
massive ($9.5 \times 10^{18} \: \mathrm{g} < M < 1.4 \times 10^{21} \:
\mathrm{g}$). These data will clarify whether such events follow existing
theories and scaling laws on solar flares and CMEs even when the energy scale
far exceeds solar cases.
