Memory effects are fascinating phenomena observed in spin glass systems. In the present study, we investigated the waiting time dependence of memory effects in the spin glass system Ni 0.45 Mn 0.55 TiO 3, whose T SG is 10 K. These results show that the memory effects are consistent with a hierarchical free energy scenario. When the system was halted at certain temperatures for longer times in the zero-field cooled process, the dip in the magnetization observed at that temperature becomes larger. A detailed analysis revealed that this change of the magnetization is well described by a stretched exponential function rather than an exponential one.