TIG arc welding is widely used because of welding various metals with high quality. In recent years, the birthrate is declining and aging. Thus, the number of technicians will decrease, and the automation of welding has become widespread. In the automation of welding, it is important to improve the moving speed of the torch and the efficiency of welding. However, the arc cannot follow the high speeding movement of the torch, and it will cause the welding defect such as a re-striking. Thus, the method of applying an external magnetic field to the arc has been studied. When the transverse magnetic field is applied, the arc is deflected by the electromagnetic force and is controlled to direction of the torch movement because of preventing weld defects. It is difficult to apply a transverse magnetic field in actual welding. Thus, we focus on the diagonal magnetic fields. In this paper, the arc deflection length affected by the diagonal magnetic field in 3D electromagnetic thermal simulation is calculated. As a result, the arc deflection length increased with increasing the magnetic flux density of the diagonal magnetic field. Also, the arc deflection length affected by the diagonal magnetic field was decreased as compared to the transverse magnetic field.