A single different-thickness plate (18) is formed by performing double-side welding from both sides in a plate thickness direction of a first front lower plate (17A) having a plate thickness equal to a plate thickness of a lower joining plate (33F) of an arm-side mounting member (33) and a second front lower plate (17B) having a plate thickness equal to a plate thickness of a third front lower plate (17C). As a result, one-side welding can be performed by using a backing material (23) from an outer side of a box-shaped structural body (12) to the lower joining plate (33F) of the arm-side mounting member (33) and to the first front lower plate (17A) constituting the different-thickness plate (18). On the other hand, one-side welding can be performed by using a backing material (26) from the outer side of the box-shaped structural body (12) to the third front lower plate (17C) joined on lower end sides of left and right side plates (13), (13′), and to the second front lower plate (17B) constituting the different-thickness plate (18).