Outdoor exposure tests and accelerated carbonation tests were conducted with the aim of ascertaining the limit of the depth at which corrosion of reinforcing steel will occur from the viewpoinit of pH gradient formed from the concrete surface accompanying the phenomenon of carbonation of concrete. Experiments were simultaneously carried out to confirm the influences of initial conditions and environmental conditions on corrosion of reinforcing steel. As a result, the following were clarified : 1. The phenolphthalein method, when using ordinary materials and under ordinary environment, is amply useful for judging limit of depth at which corrosion of reinforcing steel will occur due to carbonation. 2. The abovementioned limit seen from carbonotion is thought to be at 6 to 8mm deeper than the depth of carbonation measured by the phenolphthalein method. When chlorides are contained, it is thought the limit is 10mm farther inside. 3. Rust inhibitors used in suitable amounts for chloride content of the degree resulting from use of marine sand may be expected to provide rust preventive effects. 4. Even though slight corrosion may occur initially, the corrosion will be inhibited from progressing depending on lowering of humdity and the finishing material selected.