When a difference of electrical potentials is occurred between the inner and outer rings of rolling bearing, electrical discharge arises inside the bearing. Then, the sound and vibration, which are one of the defects of bearing, from the bearing increase in a short time. This phenomenon is called “electrical pitting”. In the raceway surface, a large linear undulation, which can be visual, is formed in the high density after the occurrence of electrical pitting. This undulation is called “ridge mark”. It is well known that the ridge mark is a typical damage morphology of electrical pitting. However, there is no report on how the ridge mark is formed on the raceway surface. Therefore, in this research, the author observed the formation process of ridge mark arising from electrical pitting experiment by using thrust bearing which is very easy to disassemble. Consequently, the following was confirmed. (1) The ridge mark did not grow evenly in all-round by little and little, but it was formed partially on the raceway. (2) Next, it spread into circumferential direction. (3) Finally, it enlarged and was formed in all-round the raceway surface.