In case of constructing the welded steel ships, the wavy deformation of the shell plate is liable to occur by the fillet welding of shell plates and frames or floor plates. This unfairness may not only spoil the appearance of the shell plate but also reduce the buckling strength of it. This study was performed for the purpose of surveying the mechanism of origination and the method of reducing this deformation. The specimen used in this experiment is a constrained frame works as shown in Fig. 1. The deformation and the reaction stress originated in the plate due to welding were measured by the dial gauge and the electric resistance wire strain gauges respectively. These measurement were performed after the welding of each layer. The results obtained are summarized as follows:- 1) Fundamental nature of this problem This problem of residual stress can be treated as the problem of reaction stress originated by constraining the angular change due to welding. Accordingly, the angular change and the reaction stress can be calculated from the measured deformation using“beam theory.” 2) Effect of welding procedures on the angular change (i) Weight of deposit metal Angular change due to welding increases as the increase of the weight of the deposit metal. There approximately exists a linear relation between the logarithm of the weight of the rod consumption and the angular change. Accordingly, it seems to be a benefit-able practice for reducing the deformation and reaction stress to decrease the weight of deposit metal or the leg length. (ii) Effect of rod diameter Size of rod diameter seems to have little effect on the angular change when the weight of deposit metal is equal. (iii) Effect of the degree of constraint Degree of constraint seems to have a remarkable effect on the angular change. Accordingly, the additional constraint, such as a kind of strong backs, attached to the shell plate may reduce the deformation and reaction stress.