High depth resolution measurements under the glancing take-off condition of medium energy ion scattering (MEIS) were carried out in order to determine the interlayer distances of the Cu/Si(111) 5 x 5 incommensurate structure. The experimental spectra were analyzed using a Monte Carlo simulation. In the simulation, the single electron excitation and the plasmon excitation were taken into consideration as the inelastic energy loss. The energy straggling was estimated by the calculation of the energy spectra for the Si(111) 7 x 7 surface, whose structure was well established. The distance between two Cu layers in the incommensurate layer was obtained to be about 0.06 nm, and that between Cu and Si layers in the incommensurate layer to be 0.006 nm. Also the distance between the Si layer in the incommensurate layer and the first Si layer of the substrate is determined to be 0.19 nm.