  We searched for superflares on solar-type stars using the Kepler
short-cadence (1-min sampling) data in order to detect superflares with short
duration. We found 187 superflares on 23 solar-type stars whose bolometric
energy ranges from the order of $10^{32}$ erg to $10^{36}$ erg. Using these new
data combined with the results from the data with 30-min sampling, we found the
occurrence frequency ($dN/dE$) of superflares as a function of flare energy
($E$) shows the power-law distribution ($dN/dE \propto E ^{-\alpha}$) with
$\alpha=1.5$ for $10^{33}<E<10^{36}$ erg. The upper limit of energy released by
superflares is basically comparable to a fraction of the magnetic energy stored
near starspots which is estimated from the amplitude of brightness variations.
We also found that the duration of superflares ($\tau$) increases with the
flare energy ($E$) as $\tau \propto E^{0.39\pm 0.03}$. This can be explained if
we assume the time-scale of flares is determined by the Alfv$\acute{\rm e}$n
time.
