Superflares are flares that release total energy 10∼10^4 times greater than that of the biggest solar flares with energy of ∼10^32 erg. We searched superflares on solar-type stars (G-type main sequence stars)
using the Kepler 30-min (long) and 1-min (short) cadence data. We found more than 1500 superflares on 279 stars from 30-min cadence data (Q0-6) and 187 superflares on 23 stars from 1-min cadence data (Q0-17)
(Maehara+2012 Nature; Maehara+2015 EPS; Shibayama+2013 ApJS). The bolometric energy of detected superflares ranges from the order of 10^32 erg to 10^36 erg. Using these data, we found that the occurrence
frequency (dN/dE) of superflares is expressed as a power-law function of flare energy (E) with the index of -1.5 for 10^33 the superflare stars show quasi-periodic light variations with the amplitude of a few percent, which can be explained by the rotation of the star with large starspots (Notsu+2013, ApJ). This interpretation is also supported by our spectroscopic studies with Subaru/HDS (Notsu+2015a&2015b, PASJ).
The bolometric energy released by flares is consistent with the magnetic energy stored around such large starspots. Furthermore, our analyses indicate that the occurrence frequency of superflares depends on the
rotation period, and that the flare frequency increases as the rotation period decreases. However, the energy of the largest flares observed in a given period bin does not show any clear correlation with the rotation
period (Notsu+2013, ApJ). In this talk, I will review these results, and discuss our future plans of superflare studies.