In order to identify the majority ionic defect of La1-xSrMnO3+delta in the oxygen-excess region, a dilatometry was made on La0.96MnO2.94+delta at a fixed temperature (1273 K) by varying the ambient oxygen partial pressure. The specimen of La0.96MnO2.94+delta expanded with increasing P-O2 in the region of delta>0, while the unit cell volume was known to decrease with increasing P-O2. The morphological evolution was also observed on the surface of the La0.96MnO2.94+delta specimen after annealing in an oxidizing atmosphere, indicating that the new lattices were created. From these results, it is concluded that the majority ionic defect of the perovskite oxide, La1-xSrxMnO3+delta, is cation vacancies in the oxygen-excess region.