The behavior of an electron in a thick Al layer is studied by the Monte Carlo method, assuming that the track of the electron is simplified by an approximate model, and the penetration and backscattering probability are obtained from a set of hundred histories. The consistency of the value of the practical range of the electron obtained by this calculation with that of the empirical formula proved that the method is quite adequate for solving the pentration and backscattering problem of the electron through a thick layer