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By extending our previous study by Maehara et al. (2012), we searched forsuperflares on G-type dwarfs (solar type stars) using Kepler data for a longerperiod (500 days) than that (120 days) in our previous study. As a result, wefound 1547 superflares on 279 G-type dwarfs, which are much more than previous365 superflares on 148 stars. Using these new data, we studied the statisticalproperties of occurrence frequency of superflares, and basically confirmed theprevious results, i.e., the occurrence frequency (dN/dE) of superflares vsflare energy (E) shows power-law distribution with dN/dE \propto E^{-\alpha},where \alpha ~ 2. It is interesting that this distribution is roughly on thesame line as that for solar flares. In the case of the Sun-like stars (withsurface temperature 5600-6000K and slowly rotating with a period longer than 10days), the occurrence frequency of superflares with energy of 10^34 -10^35 ergis once in 800-5000 years.  We also studied long term (500 days) stellar brightness variation of thesesuperflare stars, and found that in some G-type dwarfs the occurrence frequencyof superflares was extremely high, ~ 57 superflares in 500 days (i.e., once in10 days). In the case of Sun-like stars, the most active stars show thefrequency of one superflares (with 10^34 erg) in 100 days. There is evidencethat these superflares have extremely large starspots with a size about 10times larger than that of the largest sunspot. We argue that the physicalorigin of extremely high occurrence frequency of superflares in these stars maybe attributed to the existence of extremely large starspots.

Superflares on Solar Type Stars Observed with Kepler I. Statistical Properties of Superflares