Bosses extending in the boom width direction are provided in a front end part of a main boom body rotated upward, and hooks opening downward are provided in a base end part of a front boom body rotated downward. In a state that the bosses and the hooks are engaged with each other, a scooping action is performed so as to match pin holes thereof. The above hooks and the bosses are arranged so as to satisfy a condition that in a state that an angle between both boom bodies is a maximum angle at the time of work, the hooks and the bosses are moved away from each other and a distance between the hooks and the bosses is increased as the angle is reduced from the maximum angle.