The information of the transverse beam positions can be contaminated each other due to the X-Y coupling of the cavity BPM. We assumed the origin of the coupling was port's finite coupling to the orthogonal dipole mode. The outputs from each port of the cavity were measured after it was excited from a port. It was found that the X-Y coupling of the KEK BPMs and BINP BPMs were about -20 dB and -35 dB, respectively.