Indium doped Ruddlesden-Popper-type strontium titanium oxide, Srm+1(Ti1-xInx)mO(3m+1-alpha) (m = 1, 2 and infinity) were prepared and their conduction properties were investigated using various electrochemical methods. The structure of this series of oxides can be regarded as the alternative stacking of one rock-salt type layer (SrO) and m units of perovskite-type layers (mSrTiO(3)). In the cases of m = 1, the solubility limit of indium to Ti site, x, was 0.1, and the highest conductivity appeared at x = 0.07. In the cases of m = 2, Sr-3(Ti1-xInx)(x)O7-alpha, the highest conductivity appeared at x = 0.05. The order of ionic conductivities at 900 degrees C was confirmed to be SrTi0.93In0.07O3-alpha>>Sr2Ti0.93In0.07O4-alpha>Sr-3(Ti0.95In0.05)(2)O7-alpha. Under low oxygen partial pressure like in hydrogen, all samples showed n-type electronic conduction due to the reduction of Ti4+ to Ti3+. The transport number of ion in hydrogen was higher in Sr2Ti0.93In0.07O4-alpha than in others. Dominant ionic conductive species were protons in Sr2Ti0.93In0.07O4-alpha and Sr-3(Ti0.95In0.05)(2)O7-alpha but oxide ions in SrTi0.93In0.07O3-alpha. The contribution of p-type electronic conductivity under high PO2 condition was confirmed in all samples. These results suggest that the existence of a rock-salt type layer SrO in crystal structure largely suppresses the oxide ion conductivity, while p-type electronic conductivity is not suppressed by rock-salt type layer.