In a boom 15 for a working machine 10 attached to an upper rotational body 30 supported on a center of an upper portion of a crawler-type traveling device 40 to be transversely rotatable, the boom 15 includes, as portions cast by integral molding, a boom support point part 16, a boom cylinder rod support point part 18, and an arm support point part 19, and is configured so that the boom support point part 16 is connected to the boom cylinder rod support point part 18 by a general-purpose first rectangular pipe 61, and so that the boom cylinder rod support point part 18 is connected to the arm support point part 19 by a general-purpose second rectangular pipe 62.