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Some kinds of reinforced concrete structures in the natural environmental condition suffered from damages due to combined causes, i.e. chloride corrosion, carbonation, alkali-aggregate reaction (AAR), and so on. In this study, it is tried to clarify the effects of these combined deteriorating causes on the steel bar corrosion and durability of concrete structures. The first objective of this study is to investigate the effects of water cement ratio, AAR and chloride content on carbonation in concrete. The second one is to make clear the microstructure of concrete affected by combined causes. The pore volume and pore size distribution of mortar were determined by a mercury penetration method. The test results are summerized as follows;(1) The carbonation rate in concrete is not affected by the chloride content. This phenomenon is proved from the results obtained by the pore distribution measurement. (2) The carbonation rate and microstructure of concrete are affected largely by water cement ratio. (3) The chloride corrsion of reinforcing steel is accelerated by the degree of carbonation.