The fabrication accuracy of ship building block must be improved in order to raise the degree of the automation of ship built-up stage. Many research projects concerning this problem have been carried out and some practical or precise methods to estimate the welding deformation have proposed. Equivalent inherent strain method, on which the inherent strain derived from thermal cycles of welding is considered as the generating source of deformation, is a convenient method to calculate the welding deformation of large welded built-up structures. However, this method does not give a good estimation result in case of generating the large deformation such as a distortion of skin plate. In this paper, the improvement of equivalent inherent strain method to raise the precision of the distortion derived from fillet weld is carried out. Beam element, which expresses the fillet weld bead and has inherent strain generating tendon force, is incorporated with the former equivalent inherent strain method. The experimental results, which welding deformation was measured, are compared with the calculation by applying the proposed method. As a result, the validity of the improvement is confirmed.