Welding processes often induce inaccuracies for ship-building fabrication such as an excessive root gap due to the level of cutting precision and welding deformation, etc. In the Japanese ship-building industry, the decision to allow a root gap, for example, depends on the accuracy states in the Japan Ship-building Quality Standard (JSQS). The JSQS is easy to use because it gives tolerance limits and countermeasures for various issues that occur in the ship construction, such as root gap, misalignment, and angular distortion. However, the tolerance limits and countermeasures of the JSQS might be insufficient, because they still focus on the shielded metal arc welding (SMAW), which was the primary welding process used when the first version of JSQS was established around 50 years ago. The validity of widely applying the JSQS to joint welds by currently used welding processes, such as gas metal arc welding (GMAW) and laser beam welding, has not been officially confirmed and not been included in the latest JSQS. It is necessary to learn the effects of many factors, such as the amount of root gap, countermeasures, and heat input, on the static strength, fatigue strength and corrosion of ship structures when recent weld processes such as GMAW are used in order to apply the current welding processes to a new JSQS or something like it. In this study, experiments on the static strength for fillet joints are carried out by changing the amount of root gap, plate thickness, and fillet weld leg length, and the relation between the static strength and the amount of root gap is established.