The authors have already proposed the general principles in measurement of residual stresses. In this paper, the authors developed a new method for selection of observing positions of strains to ensure highly accurate measurement by using the theory of inherent strain, which is based on one of these principles. By this selection method, it is seen that the more number of measurements ensures the better average accuracy of estimated values. However, there are many positions where observed values scarcely influence the accuracy of measurement. These positions can be excluded from the measurements for reasons of economy and comparatively small number of observing positions can be selected by the present theory, which still secures a high accuracy of measurement. Furthermore, in order to measure three dimensional residual stresses in multipass welded joint, the proposed measuring method can be simplified by using the characteristics of the distribution of its inherent strains. According to this theory, three dimensional inherent strains can be divided into the components of inherent strains contained in the sliced cross section of the weld line and the longitudinal inherent strains contained in sliced plates parallel to the surface of the joint, and the measurement of three dimensional welding residual stresses has become practically possible. By this theory, the distributions of residual stresses and longitudinal inherent strains in multipass welded joint have been measured for the first time. And the estimated residual stresses show a good coincidence with the directly observed stresses on its surfaces. This implies that the present theory is reliable and applicable to measure such complex internal residual stresses.