Monte Carlo simulations were performed to examine the de magnetron discharge that creates anomalous erosion at the boundary of the curved and straight sections of a rectangular magnetron. In order to examine the cause of anomalous erosion, we performed a series of simulations describing the strong and weak field regions without the curved section. The erosion profiles were calculated under differing magnetic field distributions. We found that anomalous erosion occurs under a magnetic field of more than 200 G. We also found that the cause of anomalous erosion is the weak magnetic field at the curved section as compared to that at the straight section. We confirmed that anomalous erosion does not occur in the case of a rectangular magnetron having a uniform magnetic field. In order to avoid anomalous erosion, the uniformity of the magnetic field has to be within 10% where the vertical component of the magnetic field at the target surface is zero.