Microstructures of the high-Tc superconducting oxide found in the infinite-layer system ACuO2 (with A alkaline-earth elements) have been investigated by high-resolution electron microscopy. The superconducting samples always involve defect-layers inserted almost randomly into the parent infinite-layer structure, which we suggest are where the A cation vacancies are concentrated. Structure image observations reveal that the oxygen deficiency also occurs within a CuO2 plane in the middle of a defect layer in order to compensate for the chage imbalance due to the A vacancies. P-type superconductivity with maximum Tc of 110K is realized when the oxygen deficiency is filled in by the application of high oxygen pressure.
