In carrying out the elastic-plastic analysis, four conditions (equilibrium equation, constitutive equation, condition of compatibility and yield condition) should be satisfied. In welding, the temperature largely changed from a melting temperature to a room temperature. So, yield stress of materials largely changed, too. In particular, yield stress becomes about zero over $700^{circ}C$. The analysis should be carried out under the condition that equivalent stress generated in temperature increment ${Delta}T$ did not exceed yield stress of materials at high temperature over $700^{circ}C$. It should be sufficiently recognized that the obtained results were not reliable if this condition was not satisfied.