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We conducted high dispersion spectroscopic observations of 50 superflarestars with Subaru/HDS, and measured the stellar parameters of them. These 50targets were selected from the solar-type (G-type main sequence) superflarestars that we had discovered from the Kepler photometric data. As a result ofthese spectroscopic observations, we found that more than half (34 stars) ofour 50 targets have no evidence of binary system. We then estimated effectivetemperature ($T_{\rm{eff}}$), surface gravity ($\log g$), metallicity ([Fe/H]),and projected rotational velocity ($v\sin i$) of these 34 superflare stars onthe basis of our spectroscopic data. The accuracy of our estimations is higherthan that of Kepler Input Catalog (KIC) values, and the differences between ourvalues and KIC values ($(\Delta T_{\rm{eff}})_{\rm{rms}} \sim 219$K, $(\Delta\log g)_{\rm{rms}} \sim 0.37$ dex, and $(\Delta\rm{[Fe/H]})_{\rm{rms}} \sim0.46$ dex) are comparable to the large uncertainties and systematic differencesof KIC values reported by the previous researches. We confirmed that theestimated $T_{\rm{eff}}$ and $\log g$ values of the 34 superflare stars areroughly in the range of solar-type stars. In particular, these parameters andthe brightness variation period ($P_{0}$) of 9 stars are in the range of"Sun-like" stars ($5600\leq T_{\rm{eff}}\leq 6000$K, $\log g\geq$4.0, and$P_{0}>$10 days). Five of the 34 target stars are fast rotators ($v \sin i \geq10$km s$^{-1}$), while 22 stars have relatively low $v \sin i$ values ($v \sini<5$km s$^{-1}$). These results suggest that stars whose spectroscopicproperties similar to the Sun can have superflares, and this supports thehypothesis that the Sun might cause a superflare.

High Dispersion Spectroscopy of Solar-type Superflare Stars. I. Temperature, Surface Gravity, Metallicity, and $v \sin i$