The velocity field around a symmetrical wedge is measured with two LDVs. The data analysis of the velocity, vorticity and energy distributions is executed by using a new FFT spectrum method which can easily eliminate FM-AM modulations of the fluctuating velocity compared with the phase-average method. The analysis of these distributions based on the steady and unsteady velocity components clearly illustrates the mechanism of a Karman vortex formation in a cycle. Namely, an unsteady vortex generates just at the back of the wedge and moves in the wake. It interacts with the stationary flat vortices produced on the upper and lower, stationary but separated shear layers, supplying velocity energy to the upper vortex and removing it from the lower vortex. Consequently, the actual Karman vortex formation exhibits four stages. In the first stage the upper stationary vortex expands to the downstream direction, in the second stage it slightly rolls around the lower vortex, in the third stage it is cut off near its middle portion and in the final stage it sheds a Karman vortex.