To provide a beam member and a beam/pillar joint construction having sufficient proof stress and plastic deformation capacity even in the case of pillar/beam joint by a field welding joint system.

One end part 4 of a beam member 1 made of H-shape steel is fixed with a fixing jig 10, the one end part 4 is pressed in the lengthwise direction by pressing the other end part 4 by means of a hydraulic jack or the like while heating the one end part 4 uniformly extending over the whole territory of the vertical section by a heating device 12 up to the temperature allowing plastic deformation, hence the one end part 4 is plastically compressively deformed so as to make the wall thick, and the boundary part 6 between the thick wall end part 4 and the inside part position 5 is formed continuously tapered so as to avoid stress concentration. By joining this beam member 1 to a pillar member 7 by a field welding joint system while using together friction joint by the use of a high strength bolt 9, a plasticized territory is lengthened and sufficient plastic deformation capacity can be insured, because the flange 3 of the beam end part 4 is formed to be thick and hence sufficient bearing force can be easily obtained.