The flow patterns and the associated impulsive pressures of the Karman-vortex cavity separation from a circular cylinder have been systematically studied for several Reynolds numbers, from Re=4×105 to 7×105, by high-speed photography and by wave analysis. The main experimental features are as follows. (1) In the first period of the cavity separation, the cavity separates in a bubbly cloudform, leaving the clear part, from the boundary of the clear and cloud cavity where the transitional waves appear at about θ=110°as measured from the stagnation point. Then, the bubble cloud makes up a Karman vortex. In the second period, the residual cavity breaks down into individual bubbles of which some collapse and others make up streamwise vortex-type cavitation. (2) The impulsive pressures in the second period are about 0.6∼0.8 MPa in amplitude and 40 μs duration, so they seem to be related to the bubble collapsing pressure of low cavitation erosion. The amount is several times higher than the dynamic pressure.