Numerical simulations using finite element software were carried out to discuss the physical background of nondestructive inspections of pipes using microwaves. The simulations were conducted using an axisymmetric configuration modeling pipe with wall thinning to evaluate the effect of the profile of the wall thinning on the microwave propagation. Both rectangular and quasi-racetrack wall thinnings were considered. The numerical simulations showed the presence of wall thinning attenuates microwaves at particular frequencies. An empirical formula was proposed to evaluate the profile of wall thinning for the particular frequencies. Although the formula was based on the results of numerical simulations considering only rectangular wall thinning, it is also applicable to the evaluation of quasi-racetrack wall thinning.