A strange pulse shape has been observed in the signal of KEK cavity BPM. The pulse shape is not symmetric with respect to the cavity center, and it can not be nulled even if a beam passes its center. The observed characteristics of the signal is summarized. It has been known that there is a small coupling between the reference and the sensor cavity of the KEK BPM. We modeled the sensor cavity coupled with the reference cavity by a circuit model and simulated the signal excited by the beam. Leakage from the reference caivty can reproduce the observed phenomenon.