Temporal data of instantaneous velocity profile of a two-dimensional backward-facing step flow between separated shear layer and reattachment region are simultaneously obtained by the multi-point LDV. The details of the large-scale vortex structure at the separated shear layer and large-scale fluctuation by spatial and temporal correlation are discussed. As a result, the large-scale eddy passes frequently at whether around y/h = 0.5 or vicinity of the step wall because the different two positive peak in the spatial and temporal correlation can be observed. The instantaneous reattachment point is reduced as the vortex at the separated shear layer grows larger. The trigger for the separated shear vortex to grow larger is the reattachment of the separated shear layer. As concerning with spanwise structure, there is large three-dimensional structure in the separated shear layer at downstream of x/h = 2. On the other hand, spanwise velocity profile is similar to two-dimensional structure when the separated shear layer reattaches.