(57) A method for postweld heat treatment (hereinafter referred to as PWHT) for multilayer welding, in which the terminating point of the PWHT is judged correctly to preclude cracking due to insufficient treatment or uneconomical excessive treatment, the method comprising; determining the residual hydrogen concentration directly beneath the final welded layer immediately after completion of welding; determining a crack-preventing critical hydrogen concentration to obtain a ratio of the critical hydrogen concentration to the residual hydrogen concentration; determining the value of a product of a hydrogen diffusivity coefficient during the heat treatment and a holding time where a hydrogen concentration currently occurring in the heat treatment reaches the critical hydrogen concentration, on the basis of the relation of a ratio of the current hydrogen concentration to the residual hydrogen concentration and a sum of a parameter of hydrogen diffusion to be determined depending upon the welding conditions and the above-mentioned product; measuring the temperature of the heat treatment at a suitable point of the weld; and terminating the heat treatment at a time point when a time-integrated value of a hydrogen diffusivity coefficient at the measured temperature exceeds the value of the above-mentioned product.