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An asymptotic analysis is discussed for the radiating field when a whispering gallery (WG) mode is radiated from the edge aperture plane of a cylindrical concave conducting boundary and is propagated as a beam. The integral representing the radiation field derived by the method similar to the one used in the problem of diffraction of a plane wave from a semiinfinite conducting plane is evaluated by means of the saddle point method taking into account the existence of the poles and the residue theorem. An asymptotic representation for the beam wave is a combination of the geometric optical ray and the edge diffracted ray. The phenomenon of the radiation of a geometric optical ray from the edge aperture and the phenomenon of diffraction of the modal ray at the terminating edge are clarified. The asymptotic solutions applicable in the shadow boundary, reflection boundary, and the nearby transition region caused by the incidence of the modal ray onto the edge are compared with those obtained by the Fresnel-Kirchhoff formula so that the effectiveness of the asymptotic expression is determined. Also, it is demonstrated numerically that the radiation field expression based on the diffraction coefficients by the GTD cannot provide good results in the transition region. Further, propagation phenomena of beam fields are clarified from numerical results and the geometrical picture obtained from the ray tracing technique. [ABSTRACT FROM AUTHOR]