Slip detecting tactile sensors is essential to achieving a human-like gripping motion with a robot hand. Up until now, we have developed flexible, thin and lightweight center of pressure (CoP) sensor. The sensor, constructed of pressure conductive rubber sandwiched between two sheets of conductive film, is able to detect the center position of the load distribution and the total load. Recently, detection of initial slip has been shown to be possible. However the detection principles are unclear. Therefore, we carried out verification experiments of the slip detection properties of the CoP sensor and the detection principle. In the results, we found a change in electrical conductivity produced with a shear deformation of the pressure conductive rubber. In this paper, we will discuss the slip detection properties of the CoP sensor and detection principle.