The effect of periodic perturbation on turbulent separation from a smooth surface of an asymmetric plane-walled diffuser has been experimentally investigated. The perturbation is applied through a spanwise slit on the diffuser wall located upstream of the detachment point. The promotion of the flow reattachment is found remarkable within an optimum frequency range of the perturbation, while the flow detachment point remains unaffected. The momentum transport due to the organized velocity oscillation is explored on the basis of three-level decomposition. It is found that the production of turbulent stress is enhanced by the periodic perturbation in the separated shear layer, that occurs uniquely at the optimum frequency.