Cavitation starting at the point of separation on a smooth cylindrical wall was observed. At the earliest stage of cavitation, a cavity suddenly emerged at the point of separation with its upstream tip attached to the wall. Differing from incipient cavities arising at projected edges, the present one neither stayed nor formed a stable bubble on the wall. As it moved slightly downstream, the newly born cavity underwent severe deformation: i.e. it grew explosively, then split and ultimately vanished in reverse from the tail back, in as short a period as one ten-thousandth of a second. Flashing light was emitted from the cavity as it expanded, and a transient electric charge was also detected from the oil downstream in synchronization with this light emission. Moreover, a streamline extending from the point of separation was spontaneously visualized. In order to establish the reasons for this visualization, temperature distribution across the streamline was measured, revealing that heat has to be generated at the separation point. Finally, the mechanism of tensile stress and heat generation at the point of separation is theoretically discussed on the basis of Landau's analysis of a boundary layer flow along a separation line.