KEK cavity BPM includes a reference cavity in each BPM block. Since the distance between the reference and the sensor cavity is small, there might be a contamination of signal from the leakage of reference cavity modes. We measured the transmission from the reference cavity's port to the sensor cavity's ports and the phase relation of the transmitted signal of the ports. It seems that TM010 mode of the reference cavity couples to the TM110 mode of the sensor cavity due to the imperfect symmetry of the fabrication. Calculating the beam induced signal in the cavities, we estimated that the amount of the contamination corresponds to 30 μm position signla in the sensor cavity.