Many of the steel structures in marine environments have concrete linings which cover steel members of the tidal zone to protect the memebers from corrosion. However, little has been known on the rate of corrosion just below the lining and the conditions to cause those corrosions. It is important to have clear knowledge on these subjects. The authors carried out field investiagtions of port structures and outdoor experiments using a sea water pool. 　The following results were obtained: (1)Among 70 structures investigated, 12 structures exhibited extensive corrosions at the steel part just below the concrete lining or coping. Except one case, the structures exhibiting extensive corrosions were exposed to rivers or located close to river mouths. (2)Rete of the above mentioned extensive corrosions ranged from 0.2 to 0.5mm/y, mostly from 0.2 to 0.3 mm/y. (3)The experiments using the sea water pool showed that the macro-cell was scarcely produced at the boundary between the steel part covered by the concrete lining and the part just below the lining.