To join vertically and horizontally arranged frame members to the interior face of an outer panel, the horizontal frame members are disposed on the outer panel so as to take precedence over the vertical frame members and are weld-joined to the outer panel. The vertical frame members are placed on the horizontal frame members from the interior side and are weld-joined to the horizontal frame members. The vertical frame member includes a standard-width frame member and a wide-width frame member having a width greater than the width of the standard-width frame member. The vertical frame members are joined to and in contact with the outer panel at a reduced number points, so that the appearance of the outer panel is maintained in a good condition. With this configuration, a low cost vertical and horizontal frame structure is achieved, which has sufficiently high joint strength that provides the required strength of the outer panel.