Work Arm of Work Machine

A work arm of a work machine includes a box-shaped structure including multiple plate members, a pair of bosses attached to opposing plate members, and a cylindrical boss coupling member coupling the pair of bosses together. Each boss includes a boss body section having a pin insertion hole through which a coupling pin is inserted and extending in an arrangement direction of the plate members, and a flange section extending outward from an outer circumferential portion of the boss body section and joined to a corresponding one of the plate members via a first weld. The boss body section includes a cylindrical outer body portion on an outer surface side of the box-shaped structure and a cylindrical inner body portion on an inner surface side of the box-shaped structure. The boss coupling member is joined, at both axial ends thereof, to respective ends of the inner body portions via second welds. The inner body portion is configured such that an outer diameter at a flange section-side part thereof is smaller than an outer diameter at an axial end of the outer body portion. This allows improvement of fatigue life of the weld joining the boss and the plate member.