  23 giant flares from 13 active stars (eight RS CVn systems, one Algol system,
three dMe stars and one YSO) were detected during the first two years of our
all-sky X-ray monitoring with the gas propotional counters (GSC) of the Monitor
of All-sky X-ray Image (MAXI). The observed parameters of all of these MAXI/GSC
flares are found to be at the upper ends for stellar flares with the luminosity
of 10^(31-34) ergs s-1 in the 2-20 keV band, the emission measure of 10^(54-57)
cm-3, the e-folding time of 1 hour to 1.5 days, and the total radiative energy
released during the flare of 10^(34-39) ergs. Notably, the peak X-ray
luminosity of 5(3-9)*10^33 ergs s-1 in the 2-20 keV band was detected in one of
the flares on II Peg, which is one of the, or potentially the, largest ever
observed in stellar flares. X-ray flares were detected from GT Mus, V841 Cen,
SZ Psc, and TWA-7 for the first time in this survey. Whereas most of our
detected sources are multiple-star systems, two of them are single stars (YZ
CMi and TWA-7). Among the stellar sources within 100 pc distance, the MAXI/GSC
sources have larger rotation velocities than the other sources. This suggests
that the rapid rotation velocity may play a key role in generating large
flares. Combining the X-ray flare data of nearby stars and the sun, taken from
literature and our own data, we discovered a universal correlation of
tau~L_X^0.2 for the flare duration tau and the intrinsic X-ray luminosity L_X
in the 0.1-100 keV band, which holds for 5 and 12 orders of magnitude in tau
and L_X, respectively. The MAXI/GSC sample is located at the highest ends on
the correlation.
