Numerical simulation for the electron standing wave excited between a scanning, tunneling microscopy (STM) tip and a sample in a field emission regime has been performed using;a one-dimensional potential including a tip shape effect, It can qualitatively trace experimental results of the differential conductance (dI/dV) spectra with oscillatory peaks, which originate from the standing wave excitation. Furthermore, a band bending effect on a semiconductor surface has been evaluated including a multiple image potential in addition to a potential with a tip shape effect. By fabricating tips with a regular shape, the strength of electric field between tip and sample can be estimated from tip displacement while taking the dI/dV spectra. A band bending effect on dI/dV spectra has been evaluated to explain a parallel shift of the peaks in the spectra by the amount of band bending. We have experimentally demonstrated the shift with light irradiation to change the band bending to be flat; at least 0.4 eV upward band bending for n-type Si(001) 1 Omega cm was estimated from the peak shift. This spectroscopic method is promising to probe the electric field in the vacuum gap between tip and sample in the STM.