In order to reduce the number of fabricating processes, it is preferable to control welding distortion during welding; instead of using restraints before the process, or correcting the distortion after the process. The benefits of eliminating extra processes include the reduction of the time and manufacturing cost. This paper presents a method of back heating and welding in tandem, with the back heating occurring at a constant distance from the welding torch during welding. Traditionally, the back heating method has reduced angular distortion in two different ways; One is to bend the welded material in the opposite direction, and the other is homogenization of temperature distribution along the thickness. But, it has recently become known that the angular distortion produced by multiple heat sources, in tandem placement, is not always predicted by the total heat input to the welded joint; and it is possible for the distortion to differ greatly due to factors such as the distance between two heat sources, heat input parameters, and the heat input ratio. Based on these findings, angular distortion is expected to be reduced more effectively by choosing the proper condition for the heat source arrangement in back heating. In this paper, reduction of angular distortion by in-process control welding, using a back heating source, is numerically analyzed by the three-dimensional thermal elastic-plastic analysis, considering moving heat source with weld metal deposition. It was confirmed that the back heating method is effective in reducing angular distortion without restraint or correction. Proper condition concerning the heat source arrangement can be chosen and angular distortion can be perfectly controlled by back heating with ten percent of the welding heat input.