In order to provide a guiding principle for designing a magnetron sputtering cathode, the relationship between the erosion shape of the target and the relative electron density in the space above the target is described based on a computer simulation of electron motion. Since the most important factor is the magnetic field, the electron motion is calculated using an approximate field expression derived from the measured magnetic field. The actual erosion shape of a rectangular planar magnetron cathode, where the target is 120×560 mm in size, was compared with the calculation. The actual erosion shape coincides with the calculated relative electron density. In particular, in the area where glow discharge plasma is linear, the calculated half-value width of the relative electron density and the actual erosion halfdepth width on the nonmagnetic target are 14.5 and 15.0 mm, respectively. On the magnetic target, both are 6.0 mm.