Secondary (negative) vorticity is frequently observed at the up-wash side of the lateral vortex interacting with a wall. The generation of the secondary vorticity is found to be due to the process term of the vorticity equation in which transversal vorticity component of the lateral vortex is converted into negative vorticity by the combination of velocity gradient. In the secondary vortex region, the process term provides positive vorticity which attenuate the vorticity of the secondary vortex.