The spin polarization of secondary electrons emitted from Au thin films on an Fe substrate is studied with a Monte Carlo simulation of electron scattering. The magnetic information depth of the secondary electrons is estimated by fitting the exponential function to the calculated data and we found that the magnetic information depth increases with the primary electron energy from 11.5 at 0.3 to 26.3 Angstrom at 4 keV. This result agrees fairly with the experimental one for the same nonmagnetic/magnetic bilayer system and it is much larger than that reported so far for bulk magnetic materials.