, wherein said reinforcing means includes: a first angle portion of "L" shape in cross-section which is formed by subjecting one end portion of said tubular material to press working; a second angle portion of "L" shape in cross-section which is formed by subjecting another end portion of said tubular material to press working; and a curved intermediate tubular portion defined between said first and second angle portions, and wherein said first angle portion is fixed on and along either of said pair of vertical lateral frame portions of said tubular bus seats back frame member, while said second angle portion is fixedly connected with said support leg means.