Boom for a working machine includes a forked base support pivotally supported by a machine body. A base support of the boom has a hose introduction hole for inserting a hydraulic hose from the machine body into the boom main body. A swelling part extends upward from a fore end side toward a forked side so as to correspond to an inner surface side of an upper wall and skirt parts extend downward from a lateral center of a top part of the swelling part toward upper surfaces of left and right leg parts.