Amethod for calculating wave functions of RHEED based on a multiple scattering theory is described. Reflection matrix is used, which makes it possible to avoid divergence problem when we take account of evanescent waves as well as absorption effect. The electron density that is the square of the absolute value of RHEED wave function is calculated from Si(100)-c(4×2) surface as an example. First the change of electron density with respect to the number of diffracted beams is studied, which shows it is necessary for quantitative analysis to take many beams including evanescent beams. Then the azimuthal dependence is shown. The maximum and minimum positions of the density in the azimuth parallel to dimers are opposite to those in the azimuth normal to dimers. The density also changes as a function of the glancing angle. The maximum density is several times larger than the minimum at the same position from the surface.