A work arm for a work machine comprising: a box-shaped structure including a plurality of plate members (22, 23, 24); a pair of bosses (30A, 30B) attached to opposing plate members among the plurality of plate members (22, 23, 24); and a boss coupling member (42A, 42B) disposed between the pair of bosses (30A, 30B) to couple the pair of bosses (30A, 30B) together, the boss coupling member (42A, 42B) having a cylindrical shape, wherein each boss of the pair of bosses (30A, 30B) includes a boss body section (31A, 31B) having a pin insertion hole configured for inserting a coupling pin, the boss body section (31A, 31B) extending in an arrangement direction of the opposing plate members, and a flange section extending outward from an outer circumferential portion of the boss body section (31A, 31B) and joined, at a tip portion of the flange section, to a corresponding one of the opposing plate members via a first weld, and the boss body section (31A, 31B) includes a first body portion positioned on an outer surface side of the box-shaped structure from a position of the flange section as a boundary, the first body portion having a cylindrical shape, and a second body portion (34A, 34B) positioned on an inner surface side of the box-shaped structure, the second body portion (34A, 34B) having a cylindrical shape, and the boss coupling member (42A, 42B) is joined, at both axial ends of the boss coupling member (42A, 42B), to respective axial ends of the second body portions (34A, 34B) via second welds, and the second body portion (34A, 34B) is configured such that an outer diameter at a flange section-side part of the second body portion (34A, 34B) is smaller than an outer diameter at an axial end of the first body portion, wherein the second body portion (34A, 34B) is configured such that an outer diameter at the axial end of the second body portion (34A, 34B) is same as or larger than the outer diameter at the axial end of the first body portion, the second body portion (34A, 34B) has an annular groove formed in a flange section-side part of an outer circumferential portion of the second body portion (34A, 34B), and a diameter at a bottom portion of the groove is smaller than the outer diameter at the axial end of the first body portion. a box-shaped structure including a plurality of plate members (22, 23, 24); a pair of bosses (30A, 30B) attached to opposing plate members among the plurality of plate members (22, 23, 24); and a boss coupling member (42A, 42B) disposed between the pair of bosses (30A, 30B) to couple the pair of bosses (30A, 30B) together, the boss coupling member (42A, 42B) having a cylindrical shape, wherein each boss of the pair of bosses (30A, 30B) includes a boss body section (31A, 31B) having a pin insertion hole configured for inserting a coupling pin, the boss body section (31A, 31B) extending in an arrangement direction of the opposing plate members, and a flange section extending outward from an outer circumferential portion of the boss body section (31A, 31B) and joined, at a tip portion of the flange section, to a corresponding one of the opposing plate members via a first weld, and the boss body section (31A, 31B) includes a first body portion positioned on an outer surface side of the box-shaped structure from a position of the flange section as a boundary, the first body portion having a cylindrical shape, and a second body portion (34A, 34B) positioned on an inner surface side of the box-shaped structure, the second body portion (34A, 34B) having a cylindrical shape, and the boss coupling member (42A, 42B) is joined, at both axial ends of the boss coupling member (42A, 42B), to respective axial ends of the second body portions (34A, 34B) via second welds, and the second body portion (34A, 34B) is configured such that an outer diameter at a flange section-side part of the second body portion (34A, 34B) is smaller than an outer diameter at an axial end of the first body portion, wherein the second body portion (34A, 34B) is configured such that an outer diameter at the axial end of the second body portion (34A, 34B) is same as or larger than the outer diameter at the axial end of the first body portion, the second body portion (34A, 34B) has an annular groove formed in a flange section-side part of an outer circumferential portion of the second body portion (34A, 34B), and a diameter at a bottom portion of the groove is smaller than the outer diameter at the axial end of the first body portion.