The effect of artificially imposed periodic perturbation on the momentum transfer across the turbulent separated shear layer was experimentally investigated. The backward-facing step flow was chosen for the test case, where the perturbation was introduced as a direct suction and injection at the step edge. The velocity field was measured by a particle imaging velocimetry (PIV), which enables the observation of the large-scale vortex motion widely distributed in the separated shear layer. The data were phase-averaged, synchronizing with the perturbation, thus enabling the observation of the evolution of vortex pattern. It was revealed that the large scale vortices were introduced into the shear layer by the periodic perturbation. Investigation on the turbulent quantities based on the Reynolds averaged Navier-Stokes equation clarified that the vortex motion enhances the momentum transfer across the shear layer and promotes the reattachment.