A boom body comprises a boom front member, a boom intermediate member and a boom rear member. An arm-connection bracket is jointed to the boom front member and a vehicle body-mounting bracket is jointed to the boom rear member, thereby forming a boom. With this structure, a cross section of the boom body is less prone to be deformed and therefore, the plate thickness can be reduced, the rigidity of the boom body can be increased without mounting a cross section restraint material and the cross section of the boom is not deformed. Therefore, it is possible to reduce the boom in weight.