The reflection and transmission characteristics of a circular waveguide with coaxial-to-circular waveguide transitions are evaluated using Galerkin's method of moments. Because of the axisymmetric structure as well as the assumption of a TEM-mode incident wave, which is an axisymmetric mode, the scattered waves from the structural discontinuities can be expressed as axisymmetric modes; therefore, the analysis can be considerably simplified. In addition, by making the most of the structural symmetry for both sides (ports), a further reduction in the calculation can be performed. The characteristics are calculated as a function of frequency, and reasonable and interesting results are obtained. The circumferential slot antenna in the circular waveguide as well as the nondestructive inspection of circumferential cracks and wall-thinning inside conductive pipes are promising candidates for this technique.