By applying a mechanical load, a transient shift of oxygen potential was observed on (La,Sr)CoO3 and (La,Sr)FeO3. The theoretical relationship between mechanical stress and oxygen potential shift in ionic solids is derived using thermodynamics. The oxygen potential shift due to the mechanical stress was measured as an electromotive force (EMF) by an oxygen concentration cell. When the mechanical stress was abruptly applied, the stepwise change and subsequent decay of EMF were observed. The maximum values of the EMF essentially corresponds to those from the thermodynamic calculations. The relaxation behavior of EMF can be interpreted in terms of chemical diffusion.