A laboratory water channel experiment was made of the separated flow over a backward-facing step. The flow was excited by a sinusoidally oscillating jet issuing from a separation line. The slit was connected to a cavity in which water was forced through a rigid pipe by a scotch-yoke system. The Reynolds number based on the step height (H) was fixed at Re-H = 1200. The forcing frequency was varied in the range 0.305 less than or equal to St(H) less than or equal to 0.955 at the forcing amplitude A(o) = 0.3. Time-averaged flow measurements were made by a LDV system, especially in the recirculating region behind the backward-facing step. To characterize the large-scale vortex evolution due to the local forcing, flow visualizations were performed by a dye tracer method with fluorescent ink. The vortex amalgamation process was captured at the effective forcing frequency (St(H) = 0.477) for laminar separation. This vortex merging process enhances flow mixing, which leads to the shortening of the reattachment length.