Recently, steel has been used as leading material for constructing harbor structures in the shape of sheet pile, pipe and H-shaped pile, a good deal, so that it has become to be more important than ever to make corrosion tendency of steel at harbors clear. While, untill now, examinations regarding corrosion of steel structures at harbors have almost been conducted from point of so-called general coorosion, which was corrosion of steel buried in a single zone, for example, in sea water or in sand zone, and so on.
　However, since steel contacts nearly with several differential zones in field, for example, under marine environment, sea water and mud zone saturated with sea water, or mud zone unsaturated with sea water and mud zone saturated with sea water, and so on, it may be supposed that another steel corrosion unlike general corrosion will occure along surface of steel passing through such differential zones.
　Accordingly, to distinguish corrosion tendencies of steel in a single zone and in differential zones, in this report the former was called micro-corrosion, the latter macro-corrosion. But, the relation between micro-and macro-corrosion rates of steel has not yet been investigated quantitatively. Besides, both estimating methods of micro-and macro-corrosion rate of steel have not been developed and micro- and macro-corrosion meters, which are apparatus for estimating each micro-and macro corrosion rates under practical condition, also have not beem worked out. Consequently, to obtain a clue to the elucidation of steel corrosion problem at harbors, I started at first according to both views of micro-and macro-corrosion to arrange many data regarding steel corrosion in field once made by many engineers at many places and then to study the relation between micro-and macro-corrosion rates of steel at practical conditions and furthermore to develop estimating methods for micro-and macro-corrosion rates of steel.