A two-way relaying system using the physical-layer network coding (PNC) does not cause the spectral loss induced by the half-duplex signaling. Furthermore, when the total power, which is limited, is properly distributed to transmission nodes in such a system, the system capacity will be enhanced significantly. In this paper, we propose an optimal power allocation in the two- way relay channel employing the PNC protocol. The optimal power allocation is obtained by maximizing the achievable sum- rate of the PNC protocol under a sum-power constraint in a Rayleigh fading channel environment. Numerical experimental results confirm that the proposed power allocation scheme achieves much higher achievable sum-rate performance than the existing schemes.