IP-BPM is required to have a few nm beam position resolution and, at the same time, a beam angle insensitivity. These features are basically determined by the cavity design, especially the gap length and the beam pipe aperture. We surveyed these parameters calculating the performance of the BPM. It was found that in the vicinitiy of (gap length=5mm and pipe radius=3mm) in the parameter space was an optimum solution.