Highspeed submerged water-jets are very often successfully applied for peening and cleaning. And such a jet-working-capacity closely depends on the shedding of cavitation clouds. In order to make clear the shedding mechanism of the clouds, we systematically observe the aspects of a two-dimensional jet in a 0.35 mm-thin narrow watercourse, ejecting from the nozzle whose throat is 4 mm long and 1 mm wide. The cavitating jet is carefully observed by means of a highspeed photography and a digital image processing technique. The cavitating jet is very rapidly changing with time, and the clouds associated with the highly erosive cavitation are periodically shedding, whose frequency is about 800 Hz. The vortex cavitation around the jet is also shedding in 3 kHz. The acceleration levels associated with high cavitation-erosion-energy are also measured.