A structure for joining a cross member (2) to a side member (1) of a chassis frame, wherein a flange (4) is formed integrally at an end of the cross member via a flare (3), an extension member (5) inserted into the end of the cross member (2) to project from the afore-mentioned flange (4) is welded to the flare (3), the extension member (5) is passed through an inside hole (9i) formed in an inside part (8i) in the width direction of the vehicle in the closed-section part of the side member (1) and welded to an outside hole (9o) formed in an outside part (8o) in the width direction of the vehicle, and the outer periphery of the flange (4) is welded to the inside part (8i) in the width direction of the vehicle. The closed-section part (8) is formed in a rectangular shape, and the top and bottom parts of the periphery of the flange (4) are located near top and bottom bends (12) of the closed-section part (8).