Transcranial magnetic stimulation (TMS) allows non-invasive and painless stimulation of local cerebral nerves using eddy current generated by electromagnetic induction with a TMS coil. Although TMS is used in various fields, which area of the brain is stimulated is not known because of the complicated structure of the organ. In this study, we simulated neuronal excitement by TMS using the finite element method. First, we designed a brain sulcus model consisting of cerebrospinal fluid, gray matter and white matter, using 0.5 mm cube elements. To improve calculation accuracy, cube element size was set to 0.5/3 mm only in regions near the boundary surface. Second, we applied TMS stimulation to the model in different conditions. We used coil radii of 10, 20, 30 and 40 mm, and coil orientation at 0°, 30°, 45°, 60° and 90°, which is defined as the angle between the orientation of the electric field and the axon. Finally, we calculated the membrane potential and compared the results obtained under different conditions. We found that membrane potential changed rapidly at the white matter and gray matter interface when the coil radius was over 20 mm and coil orientation was within 60° between the orientation of the electric field and the axon. These results provide useful information on appropriate TMS parameters for effective stimulation of target area in the brain.