Cavity BPMs to be used in ATF2 have an artificial frequency split in dipole modes of x and y directions. The frequency split is realized by two dents placed on one axis. It is expected to align the axis of eigen modes. We confirmed the effectiveness of the dents design by a simulation. We calculated xy isolation with a model which has an error at undesireble position. It was found that factor 10 stronger effect than the error is needed to realize the target value of isolation.