The authors deals with an instability created on a flat-plate shear layer by a finite-core vortex convecting outside the shear layer. In the experimental study it is shown that a vortex with positive criculation creates instability ahead of it. Numerical simulation is performed for a case similar to the experiment. Moreover, the coupling between the convecting vortex outside the shear layer and the generated unsteady vortical field inside is explained theoretically by deriving an equation for the total mechanical energy. Finally, the unsteady vortical field created in the computation is analysed by proper orthogonal decomposition, which supports the proposed mechanism.