Mechanical phenomena in multi-pass welding can be separated into that of newly welded metal and existing weld metal under repeated thermal cycle. In this report, the latter is investigated using FEM, simple mechanical model and experiment. The mechanism of inherent strain formation under repeated thermal cycle is clarified using constraint and heat input as governing parameters. The inherent strain is the non-elastic part of the strain which is considered to be the cause of both welding deformation and residual stress. Thus, the welding deformation and residual stress can be computed by elastic FEM if the inherent strain is known. The procedure to estimate the inherent strain for the multiple thermal cycle is proposed and its usefulness is demonstrated through a numerical example.