In Physical Layer Network Coding (PLNC), since the two sources access the relay station at a time, the relay station can hardly demodulate each information owing to the mutual interference between two transmitted signals. The PLNC achieves the high secure relay communication based on physical layer security (PLS). The source needs the transmit power control based on the channel state information (CSI) for maintaining the secure mutual interference. The untrusted relay, however, may impersonate the CSI for breaking it. So far, the authors have evaluated the amount of the leaked information to the untrusted relay by the impersonation of CSI but the leaked information is not maximal. For the countermeasure to the impersonation of CSI, the most powerful model of the untrusted relay to maximizing the leaked information is necessary. This paper constructs the optimal impersonation of CSI by a linear programing problem.