Temperature and magnetic field dependences of the critical current density Jc and the averaged pinning potential Up, were measured in order to understand the mechanism of the irreversibility line in high-T, oxide superconductors by the use of an ac inductive method. The flux motion in the pinning potential well in the vicinity of the irreversibility temperature Tirr, was also investigated. As the temperature increases to Tirr s Up decreases nearly equal to the thermal activation energy kBTirr. This result suggests that the irreversibility line is the thermally induced depinning line.