this welded structure comprises a weld metal which contains c , si , mn , cr , mo , v , nb , n and o in prescribed amounts respectively with the balance being fe and unavoidable impurities and which exhibits an a value of 200 or more and a z value of 0 . 05 or more . the a value is calculated from the element contents of the weld metal according to the formula : a value =/ 51 +/ 93 )/×/ 5 +/ 2 )}× 10 4 . the z value is calculated according to the formula : z value = n × is the number density of carbide particles present in a prior austenite grain boundary per unit grain boundary in the stress - relief annealed weld metal , and is the concentration of compound - type v as determined by analyzing an extraction residue of the stress - relief annealed weld metal ].