In this work, the Finite Element Method (FEM) is applied in the analysis of electromagnetic field generated by instruments employed for inspection of pipelines for natural gas-transmission. Inspection instruments can use this method to evaluate anomalies as corrosion and kneading in the superficies of the pipelines. This can avoid leaking that can cause ecological accidents. The Magnetic Flux Leakage (MFL) is the oldest and most commonly used in-line inspection method for finding metal-loss regions in gas- transmission pipelines. MFL inspections are typically used to detect, locate, and characterize metal-loss and other anomalies in natural gas-transmission pipelines. The amplitude or magnitude of an MFL signal is strongly related to defect depth. Alternatively, in this work, an instrument that generates a TEM wave is explored for inspection of pipelines. To evaluate the field distribution inside the pipe, an algorithm based on Finite Element Method is used to detect and locate an anomaly in the pipeline. It is used two kinds of source in the duct´s port: (I) just propagation in the FEM basic mode and (II) different potential between the internal and external cylindrical duct. Several sizes of anomalies will be analyzed to identify for which kind of problems the algorithm is useful.