High electric fields at ZnSe/GaAs heterointerfaces grown by atmospheric-pressure organometallic vapor phase epitaxy (OMVPE) were revealed by fast Fourier-transformed photoreflectance (FFT-PR) measurements at room temperature. In the PR measurements, a modulation light was provided by a He-Ne laser exciting only GaAs regions at the interfaces. PR spectra were obtained with Franz-Keldysh oscillations (FKOs) above the band-gap energy of GaAs. FFT was successfully applied to the PR spectra to calculate the interface electric field from the FKOs. The observed electric fields were very high and in the range of 100-150 kV/cm. The dependence of the electric fields on the Zn- or Se-exposure duration showed a clear relationship with that of the valence-band offsets determined by X-ray photoemission spectroscopy (XPS). The resultant electric field was discussed in conjunction with the valence-band offsets.