To provide the boom structure of a hydraulic excavator, in which a reinforcing plate is disused by increasing the strength of a welding section in a boom curved section and lightening a boom.

An upper plate 2, a lower plate 3 and a side plate 4 are combined in a box-shaped cross section, and a boom 1 curved in a chevron shape in the longitudinal direction is formed by welding the connecting sections of each plate. A J-shaped groove section 4a is fabricated to at least either one of a side-plate upper edge section or a side-plate lower edge section in a boom curved section at that time, and the boom 1 is formed by conducting T-joint welding to the side plate 4.