A pivoted coupling member of a two-piece boom can be shared with standard booms. The boom comprises a first boom having, on the base end side, a pivoted coupling member that is pivotably coupled to a boom support section, and a second boom in which the base end side is pivotably coupled to the distal end side of the first boom and in which an arm is pivotably coupled to the distal end side. In the boom, the lateral width of the main body portion 51 of the second boom (18B) is designed to be smaller in the second region (Y) of the base end side than the first region (X) of the distal end side, the base end side of the second region (Y) of the second boom (18B) in inserted between the left and right side walls (29) of the distal end side of the first boom (18A), and the second boom (18B) is pivotably coupled to the first boom so as to be capable of rotating about the lateral axis.