Cavitation occurring at a sharp projection in a hydraulic oil flow was observed, using a microscope, video-cameras, laser beam and a photo-multiplier. On a sewing needle employed as a projection, a tiny cavity as small as several 10 μm in length suddenly emerged at its tip and stayed afterwards. In vigorous cavitation the needle emitted light and simultaneously produced synchronized electric charge. At the same time the electrode placed downstream of the needle detected the electric charge of an opposite sign. The observations by the microscope made it possible to determine the exact start of cavitation, which turned out to depend on the oil temperature as well as the downstream pressure. Moreover the streamline stretching from the tip of the needle became visible as the flow rate increased. There was evidence that the highly viscous shear flow generated heat at the separation point, which probably explains why the streamline was visualized.