Photoreflectance (PR) of surface-intrinsic-n + type doped GaAs has been measured for various power densities of pump laser. The spectra exhibited many Franz-Keldysh oscillations, whereby the strength of electric field F in the undoped layer can be determined. The thus obtained Fs are subject to photovoltaic effect and are less than built-in field F bi. In the previous work we have obtained the relation F≈F bi-δF/2 when δF≪F bi by using electroreflectance to simulate PR, where δF is the modulating field of the pump beam. In this work a method was devised to evaluate δF by using photoinduced voltages V s and, hence, the relation can be verified by PR itself. The δFs obtained by V s are also consistent with those of using imaginary part of fast Fourier transform of PR spectra.