In left and right web plates (12) of a boom (11), first to fifth web materials (12A to 12E) are joined at edge portions in a front-rear direction along welding lines (13A to 13D). In an upper flange plate (14), a rear upper flange material (14A), a front upper flange material (14B), and a middle upper flange material (14C) are joined at edge portions in a front-rear direction along welding lines (15A, 15B). Ina lower flange plate (16), first to sixth lower flange materials (16A to 16F) are joined at edge portions in a front-rear direction along welding lines (17A to 17E). The welding lines (13A to 13D) of the left and right web plates (12) are disposed in positions that are different from those of the welding lines (15A, 15B) of the upper flange plate (14) and the welding lines (17A to 17E) of the lower flange plate (16) in a front-rear direction. This makes it possible to reduce the entire weight and thereby achieve reduction in weight while ensuring bending strength, torsional strength, and the like of the boom (11).