When forming unipolar resistance switching in material systems, fatal damage often occurs to the top electrodes of Pt/SrTiO x/Pt, Pt/TiO y/Pt and Pt/NiO z/Pt cells. To develop a means of overcoming this problem, we systematically investigated the forming process by applying triangular-and pulse-waveform voltage signals to the cells. By investigating the dependence on sweep rate of the triangular-waveform voltage signals and amplitude of the pulse-waveform voltage signals, we discovered that the forming process occurred by two different mechanisms, irrespective of the material: either a thermally assisted dielectric breakdown or a purely electrical dielectric breakdown. During the former process, the top electrodes remained clean, even though oxygen bubbles formed on them. We observed that the top electrodes were blown off only for the latter (electrical) breakdown as a result of the formation of many conducting channels. We were able to overcome the fatal damage to the top electrodes by modifying the forming process into the thermally assisted dielectric breakdown.