An experimental study was made of a large-scale vortical structure over a backward-facing step. The Reynolds number based on the step height was R $e_{H}$ =33,000. To recognize the large-scale vortex, three components of velocity were measured. The measurements were performed in the recirculation zone (x/H=4.0) and the reattachment zone(x/H=7.5). To measure the wall pressure fluctuations in a turbulent flow over a backward-facing step, a 32-channel microphone array was installed beneath the wall in the streamwise and spanwise directions. From the measured pressure field, the size of large-scale vortex was obtained. As a detailed study, a conditionally-averaging technique was employed to characterize the coherent structure of the large-scale vortex. To see the relationship between the flow field and the relevant spatial mode of the pressure field, the spatial box filtering (SBF) was examined. A cross-correlation between velocity and pressure fluctuations was performed to identify the structure and the length scale of the large-scale vortex.x.