PURPOSE:To make up a boom with good welding efficiency without the use of reinforcing plates and partition walls by a method in which left- and right- handed cylinder brackets with webs are connected with each other by a lower flange, between the left- and right-handed brackets is welded, and then an upper flange is welded to the webs. CONSTITUTION:A left-handed boom cylinder bracket 11 is welded to a left web 10, and a right-handed boom cylinder bracket 13 is welded to a right web 12. A lower flange 14 is welded to the left and right webs 10 and 12, and a lower- side intermediate boom cylinder bracket 15 of a half-split form is welded to the inner end portion of the left- and right-handed boom cylinder brackets 11 and 13. An upper-side intermediate boom cylinder bracket 16 of a half-split form is welded to the left- and right-handed boom cylinder brackets 11 and 13 and the lower-side intermediate boom cylinder bracket 15, and an upper flange 18 is welded to the left and right webs 10 and 12. An inexpensive boom of good welding reliability can thus be obtained.