The solid solutions of oxides; A : SrLaFeO_(4-y) having a K₂NiF₄-type layer structure, and B LaFeO_(3-y), C : La_(0.8)Sr_(0.2)FeO_(3-y), D : SrFeO_(3-y), E : Sr_(0.9)La_(0.1)FeO_(3-y) with perovskite structures have been prepared at 1200℃ under the atmospheric air pressure. Nonstoichiometric chemical formulas have been formulated from the data such as τ(amount of Fe^(4+) ion) and y (oxygen deficiency) values from Mohr salt analysis. Electrical conductivity measurements performed in the temperature range 283∼1173 K under atmospheric air pressure shows that sample B containing nearly 100% trivalent iron is an insulator, whereas the other samples (τ≠0) show semiconducting behaviour. Sample A with typical tetragonal K₂NiF₄ structure shows a transition point at 363 K. At that point the log conductivity value increases from -2.20 to -1.71. Samples C, D, E have their peaks at 643K, 583K, 723K, respectively and thence semiconductivity changes into metallic property.