We investigated the origin of retention failure in ferroelectric memories (FeRAMs) with SrBi2(Ta, Nb)2O9 (SBTN) memory cell capacitors by considering the time-dependent behavior of polarization vs. voltage (P-V ) curves of the capacitors during high-temperature storage. Since the SBTN capacitors exhibited no marked decrease in the nonvolatile component of polarization even after high-temperature storage, we focused on the effect of voltage shift observed in P-V curves. We calculated bitline voltage along the storage from the P-V curves and the bitline capacitance, and successfully estimated a decrease in the bitline voltage, which is in agreement with the retention failure in FeRAMs. In addition, the calculation indicated that the lifetime limited by the retention failure in FeRAMs with SBTN capacitors at 125°C exceeds 10 years.