The electroreflectance spectra of surface-intrinsic- n+-type-doped GaAs were measured at various bias voltages (Vbias). Results revealed many Franz-Keldysh oscillations (FKOs) above the band-gap energy, which have been attributed to a uniform electric field (F) in the undoped layer below the surface. However, there has been no other evidence for the uniformity of F in the undoped layer. Since it is known that F can be deduced from the periods of the FKOs, the relations between F and Vbias can, thereby, be obtained. The nearly linear relation, thus found, confirms the existence of a nearly uniform field in the undoped layer. From the plot of F against Vbias, the values of the thickness of the undoped layer and the barrier height can also be evaluated.