An arm of a hydraulic excavator is formed as a box-shaped structural body surrounded by left and right side plates, an upper plate joined to each of the upper end sides of each side plate, a lower plate joined to each of the lower end sides of each side plate, and a thick rear plate joined to each of the rear end sides of each side plate and the upper plate. The side plates are formed by joining a rear thick side plate and a front thin side plate. The upper plate is formed by joining two members, that is, a rear thick upper plate and a front thin upper plate, and the lower plate is formed by joining two members, that is, a rear thick lower plate and a front thin lower plate.