  Monitor of All-sky X-ray Image (MAXI) detected a superflare, releasing
$5\times 10^{37}$ erg in 2$-$10 keV, of the RS CVn-type star IM Peg at 10:41 UT
on 2023 July 23 with its Gas Slit Camera (GSC; 2$-$30 keV). We conducted X-ray
follow-up observations of the superflare with Neutron Star Interior Composition
ExploreR (NICER; 0.2$-$12 keV) starting at 16:52 UT on July 23 until 06:00 UT
on August 2. NICER X-ray spectra clearly showed emission lines of the Fe XXV
He$\alpha$ and Fe XXVI Ly$\alpha$ for $\sim 1.5$ days since the MAXI detection.
The Fe XXV He$\alpha$ line was blue-shifted with its maximum Doppler velocity
reaching $-2200 \pm 600$ $\mathrm{km \: s^{-1}}$, suggesting an upward-moving
plasma during the flare, such as a coronal mass ejection (CME) and/or
chromospheric evaporation. This is the first case that the Fe XXV He$\alpha$
line is blue-shifted during a stellar flare and its velocity overwhelmingly
exceeds the escape velocity of the star ($-230$ $\mathrm{km \: s^{-1}}$). One
hour before the most pronounced blueshift detection, a signature of reheating
the flare plasma was observed. We discuss the origin of the blueshift, a CME or
high-velocity chromospheric evaporation.
