The Cu ion migration behavior of a Pt/Cu/HfO2/Pt structure, which is a high-k dielectrics based resistive random access memory (ReRAM) and exhibits resistance switching behavior, against the sweep number of pulsed bias voltage was investigated by hard Xray photoelectron spectroscopy (HX-PES). A forward bias application induced the reduction of the unintentionally oxidized Cu top electrode to the Cu ion migration in the HfO2 layer. resulting in the switching from the high resistivity to the low resistivity at approximately ±1.2 V. HX-PES also indicated chat the intensity ratio of Cu 2p3/2 to Hf 3d5/2 core level spectra decreased with increasing the sweep number of pulsed bias voltage. suggesting that Cu moves to the bottom Pt electrode side continuously, which consists with the degradation of switching properties. The Cu ion has a key role in the switching of the Pt/Cu/HfO2/Pt structure.