We derive a novel high-frequency asymptotic solution for scattered fields by a junction of planar impedance surfaces assuming that the transmitting and the receiving antennas are placed sufficiently away from the impedance surfaces. An integral representation for scattered fields derived by using the aperture field method is evaluated asymptotically by applying the saddle point technique applicable uniformly as the saddle point approaches the endpoint of the integral. The novel asymptotic solution includes the higher-order term. By comparing with the reference solution calculated by the numerical integration of the integral, the validity of the asymptotic solution is confirmed.