The detail structure of heavy rust layers with large swelling and laminated layers formed on weathering steel bridges have been investigated by optical microscope and laboratory powder X Ray diffraction (XRD) and synchrotron XRD in SPring 8. Some large gaps (inter layer) and many linearly arrayed voids (LAV) were found along layers. The mass ratio of spinel type iron oxide [mainly Magnetite (Fe 3 O 4 )] in average composition of the whole layer was 30 40 mass. In contrast the mass ratio of spinel in its local parts, i.e. outer layer, inter layer and inner layer position was not higher in common but mass ratio of b FeOOH was higher. Thus we propose a multilayer model structure for these unique rust layers which are made of Spinel Poor, Rich and Poor unit cell structure (SPRaP cell) compartmentalized by LAV.