In order to increase the productivity of shipbuilding to respond to current situations such as decreasing numbers of skilled workers and aging of workers in the shipyard, mechanization and robotization have been accelerated. To apply them effectively, the current fabrication and assembling system of ship hull has to be reviewed so that the dimensional accuracy of the hull block and its components increase particularly. Accordingly, the authors had developed an "accurate production system" for the fabrication and assembly of flat block, which was based on advanced accuracy technology and modem automated machines. This report concerns the deformation due to the filet welding of longitudinals on a skin plate and its straightening; the estimation of welding deformation, especially angular distortion, and its straightening method using a press roller which was developed newly in this automated production line. In order to clarify the power of the straightening machine, non-linear FEM analysis was carried out.